Catalysts For Dehydrogenation Of Ethylbenzene Research Articles

Alumina-boron catalysts for oxidative dehydrogenation of ethylbenzene to styrene: Influence of alumina-boron composition and method of preparation on catalysts properties

Different mole ratio Al-B catalysts (Al-10B to Al-35B) were synthesized by using sol-gel (SG) method. Ethyl benzene (EB) dehydrogenation in the presence of oxygen and water steam was carried out on these catalysts at 450–500 °C with EB contact time of 0.54 gcat.s.cm–3. Acidity of Al-B catalysts was estimated by using NH3-TPD-mass spectral analysis studies. SEM-mapping images revealed fine distribution of boron up to 15% of its loading in alumina (Al-15B), whereas, boron aggregation was observed in higher boron content (Al-25B and Al-35B) catalysts. Essentially, acid sites of very weak strength (Tmax ≤ 125 °C) were observed for Al and Al-10B catalysts and resulted in low EB conversion and styrene yield. On the other hand, acid sites of weak strength (Tmax ≤ 180 °C) were observed for Al-25B and Al-35B catalysts and resulted in high EB conversion. However, greater styrene yield (43.2%) with reasonable EB conversion (46%) was obtained on acid sites of weak moderate strength in Al-15B catalyst. Further, Al-15B catalyst was synthesized by using co-precipitation (COP) and impregnation (IMP) methods. Acid sites related to NOx formation during the NH3-TPD-mass analysis on IMP and COP catalyst essentially improved the EB conversion to 66% and 63% respectively at 500 °C. However, these acid sites were diminished in Al-B SG catalyst and resulted in 50% of EB conversion at 500 °C. At 50% of EB conversion level, styrene selectivity of 73%, 82.5% and 84% were observed on Al-B IMP, Al-B COP and Al-SG catalysts, respectively. Hence, different method of preparation of Al-B catalyst generated acid sites of different strength and density and thereby influenced the styrene formation.

Understanding the active-site nature of vanadia-based catalysts for oxidative dehydrogenation of ethylbenzene with CO2 via atomic layer deposited VOx on γ-Al2O3

Supported vanadium oxides are widely used as active catalysts for a number of oxidation and oxidative dehydrogenation reactions, and the understanding the structure-function relationship is of great importance for developing more efficient vanadium oxide catalysts. In this work, atomic layer deposition (ALD) was utilized to disperse sub-monolayer VOx over a commercial γ-Al2O3 support. Characterization results indicate that highly dispersed VOx species with different structures were obtained by simply changing the number of ALD cycles. The vanadium oxides synthesized by performing 1 cycle of ALD were exclusively isolated monomeric VOx. With increasing the number of ALD cycles from 3 to 8, the polymeric VOx spices were formed, and the polymerization extent or domain size of VOx was increased. On the contrary, crystalline V2O5 besides polymeric VOx was found if the number of ALD cycles was extended to 12. Catalytic performances of ALD VOx/γ-Al2O3 were evaluated for the oxidative dehydrogenation of ethylbenzene with CO2 (CO2-ODEB), and the isolated monomeric VOx species were found to be more active and more stable than the polymeric VOx species and crystalline V2O5. By correlating the specific activity and the structural characteristics of ALD VOx/γ-Al2O3 catalysts, the VOAl bonds were concluded as the key active sites for CO2-ODEB.

Defect-mediated hydroxylation of multi-walled carbon nanotubes as metal-free catalysts to enhance catalytic performance for oxidative dehydrogenation of ethylbenzene using CO2

The fabrication and hydroxylation of surface defects on multi-walled carbon nanotubes (MWCNTs) are performed through a single or combined method involving hydrothermal, ultrasonic and ball-milling treatments. These treatments cannot change the tubular morphology of the graphite structure in MWCNTs but they can alter the surface defect structure on MWCNTs. Surface defects on MWCNTs can promote the hydroxylation of the MWCNTs to produce hydroxyl groups. However, these defects are difficult to hydroxylate when occupied by other functional groups, such as carboxyl groups, even though the hydroxylation processes are the same. After hydroxylation, the MWCNTs with surface defects can be employed as metal-free catalysts for the oxidative dehydrogenation of ethylbenzene using CO2 as soft oxidants. The inherent disorder or artificial skeleton defects on MWCNTs cannot directly act as catalytic active sites, but they can be hydroxylated to enhance the catalytic activity of the MWCNTs. For the same type of MWCNT defects formed after hydroxylation, the catalytic activity is closely related to the defect density. If the disorder or skeleton defects cause a higher defect density on the hydroxylated MWCNTs, the MWCNTs will exhibit a higher catalytic activity. Due to the different structures and positions of the MWCNT defect carbons, the artificial skeleton defects on MWCNTs more efficiently promote the hydroxylation of MWCNTs compared to the disorder defects, decreasing their defect density and enhancing their catalytic activity. Therefore, MWCNTs-BOH36 with skeleton defects has a catalytic activity that is twice as high as that of MWCNTs-CS-120 with disorder defects.

Nanodiamond@carbon nitride hybrid with loose porous carbon nitride layers as an efficient metal-free catalyst for direct dehydrogenation of ethylbenzene

This work presents a facile and low-cost approach to prepare nanodiamond@carbon nitride hybrid with loose porous carbon nitride layers close wrapped on well dispersed NDs (ND-CN) through the thermal polymerization of the mixture composed of hexamethylenetetramine (HTM) and ammonium chloride adsorbed on dispersed nanodiamonds (ND), in which hexamethylenetetramine serves as both carbon nitride precursor and dispersing agent and the ammonium chloride acts as gas template. ND-CN shows 1.9 and 1.2 times much higher styrene rate for oxygen and steam-free direct dehydrogenation of ethylbenzene than the pristine ND and the previously reported carbon nitride layers encapsulated ND (H-ND) formed by the thermal polymerization of mixture composed of HTM and ND, respectively, ascribed the more exposed active sites containing structural defects and ketonic carbonyl groups and the not weakened synergistic effect of carbon nitride with ND that originating from the enlarging surface area and pore volume owing to the characteristic concerning loose porous structure of carbon nitride layers.

La2O3 promotional effect to Co3O4/γ-Al2O3 catalyst in the oxidative dehydrogenation of ethylbenzene with CO2 as soft oxidant

Co3O4/γ-Al2O3 catalysts with variable Co3O4 loadings (5–20 wt%) and deposition of 15% Co3O4 on La2O3/γ-Al2O3 were prepared by wet impregnation method. La2O3-γ-Al2O3 support with variable composition of La2O3 (2–6 wt%) were prepared by co-precipitation method. All the catalysts were tested for oxidative dehydrogenation of ethylbenzene with CO2 as soft oxidant. Among the Co3O4/γ-Al2O3 catalysts, 15% Co3O4/γ-Al2O3 has shown good performance and hence this catalyst has been chosen to investigate the effect of La2O3 species. CO2 pulse chemisorption data indicate more amount of CO2 uptake over 15% Co3O4/4%La2O3/γ-Al2O3 catalyst which clearly indicates that this catalyst exhibits good performance in ethylbenzene dehydrogenation with CO2 as soft oxidant because of reverse water gas shift reaction. Temperature programmed reduction studies indicate that the Co3O4 catalysts follow two step reduction mechanism from Co3O4 to CoO and then to Co and La2O3 promotional effect is visible through facile reduction of Co3O4 species. La2O3 doping has a vital influence in getting enhanced ethylbenzene conversion, styrene yield and alleviates catalyst deactivation compared to that of unpromoted Co3O4/γ-Al2O3 catalyst. TGA studies indicate the presence low amount coke deposition during time-on-stream over 15% Co3O4/4%La2O3/γ-Al2O3 catalyst compared to 15% Co3O4/γ-Al2O3 catalyst.

Defect-rich Ce1-xZrxO2 solid solutions for oxidative dehydrogenation of ethylbenzene with CO2

The zirconium doped ceria (Ce1-xZrxO2, x = 0.0, 0.1, 0.3, 0.5, 0.9, and 1.0) was synthesized via the urea-hydrolysis assisted hydrothermal method by using (NH4)2Ce(NO3)6 and ZrO(NO3)2·2H2O as the precursors. The structural and textural properties of the oxides were characterized with pertinent techniques, and the materials were evaluated as catalysts for the oxidative dehydrogenation of ethylbenzene with carbon dioxide (CO2-ODEB) under the conditions of 550 °C, 0.1 MPa, CO2 to ethylbenzene molar ratio of 20, and W/F = 8.07 g-cat·h∙mol−1. Characterization results indicate that the single-phase cubic solid solution was formed in the cases of Ce0.9Zr0.1O2, Ce0.7Zr0.3O2, and Ce0.5Zr0.5O2. In contrast, separated tetragonal and monoclinic phases of ZrO2 were present over Ce0.1Zr0.9O2 besides the solid solution. Moreover, the relative content of defects over the surface of Ce1-xZrxO2 was continuously increased with x, but the highest amount of reducible Ce4+ was obtained over Ce0.5Zr0.5O2. Reaction results indicate that Ce1-xZrxO2 was effective for CO2-ODEB, and the highest conversion of ethylbenzene was obtained over Ce0.5Zr0.5O2. The activity of Ce1-xZrxO2 for the titled reaction is determined by both the amount of the reducible Ce4+ and the easy oxidation of Ce3+ by CO2 over the catalyst. The inhibition of the coke via the oxidation of Ce3+ by CO2 and its promotional effect on the activity are responsible for the stability of the catalyst for CO2-ODEB.

Ordered mesoporous alumina-supported vanadium oxides as an efficient catalyst for ethylbenzene dehydrogenation to styrene with CO2

Ordered mesoporous alumina (OMA)-supported vanadia catalysts with different vanadium loading (denoted as nV/OMA) were employed for ethylbenzene dehydrogenation to styrene with CO2 as soft oxidant, and a commercial γ-Al2O3-supported vanadia catalyst for comparison. The catalytic behavior depends strongly on the vanadium loading and the support properties. With an optimal V loading of 0.8 mmol/g-support, 0.8 V/OMA catalyst exhibits superior catalytic performance than that of 0.8 V/γ-Al2O3, which can be attributed to the excellent structural, textual, and surface properties of OMA support. The important roles played by OMA lies in stabilizing highly dispersed V5+ active species and preventing the coke formation.

Catalytic behavior of manganese oxides for oxidative dehydrogenation of ethylbenzene with carbon dioxide

Manganeses oxides (MnOx) with different crystal and structural properties were prepared by the precipitation, the complex-decomposition, and the thermal decomposition methods, respectively. The materials were characterized by XRD, SEM, TEM, H2-TPR, TG-DSC, XPS, and N2 adsorption/desorption at low temperatures. The samples were comparatively evaluated as catalysts for the oxidative dehydrogenation of ethylbenzene with carbon dioxide (CO2-ODEB). Results indicate that both Mn2O3 and Mn3O4 were active for CO2-ODEB and the Mn3O4 prepared by the precipitation method showed the highest conversion of ethylbenzene over 60%. Moreover, the activity of MnOx for CO2-ODEB was significantly affected by the synthesis method and parameters of the oxides. The correlation between the reaction and characterization results indicates that the CO2-ODEB over MnOx may follow the redox mechanism involving the lattice oxygen, and the amount of the mobile oxygen species and the surface area were revealed to be key factors in determining the activity of the MnOx catalysts for the titled reaction. Together with the characterization results of XRD, TG-DSC, and H2-TPR, the irreversible reduction of MnOx to MnO under the conditions of CO2-ODEB was the main reason for the deactivation of the catalyst while the coke deposition had a less important impact on the stability of the catalyst.

Few-layer sp2 carbon supported on Al2O3 as hybrid structure for ethylbenzene oxidative dehydrogenation

Hybrid materials consisting of few-layer sp2 carbon supported on Al2O3 can potentially display comparable performance with nanodiamonds for the oxidative dehydrogenation of ethylbenzene. The hybrid structure could be easily prepared by fast coking method using ethanol as a precursor and ɣ-Al2O3 with high specific surface areas as a template. In this case, ɣ-Al2O3 plays the role of 3D support for depositing few-layer graphene-like structure. The hybrid structure exhibiting similar structure with nanodiamonds calcined at high temperature is further used as a metal-free catalyst for the oxidative dehydrogenation of ethylbenzene to styrene. The yield rate of styrene normalized by the specific area on Al2O3@C is 0.044mmolm−2h−1 (TOF=4.13h−1) which is two times of that on nanodiamonds (0.020mmolm−2h−1, TOF=3.27h−1). Considering the low cost process and good catalytic performance, the hybrid nanostructure may be a promising candidate to be applied in the oxidative dehydrogenation of ethylbenzene.

Development of stable MoO3/TiO2-Al2O3 catalyst for oxidative dehydrogenation of ethylbenzene to styrene using CO2 as soft oxidant

A novel MoO3/TiO2-Al2O3 (MTA) catalyst was employed for oxidative dehydrogenation of ethylbenzene to styrene using CO2 as a soft oxidant with CO2/ethylbenzene (molar ratio)=9.8 and gas hourly space velocity (GHSV) was maintained at 0.08molh−1g−1catalyst. Among different (2.5, 5.0, 7.5 and 10.0) wt.% loadings of MoO3, 7.5wt.% loaded MoO3/TiO2-Al2O3 catalyst exhibited superior activity. The stability of the catalyst over time on stream was noteworthy where; the catalyst evinced notable activity up to 40h with maximum ethylbenzene conversion of about 70% and styrene selectivity about 97%. The presence of titania greatly influenced the activity of the catalyst in terms of stability. The reaction with CO2 resulted in better activity compared to N2 proving it to be an effective oxidant for the dehydrogenation reactions. The deactivation of the catalyst was due to excess coke formation which led to blocking of active sites.The coke was removed by regenerating the catalyst at 600°C under air. Interestingly, the catalysts were found active even after 40h as well as in between 10h of regenerations. The catalysts were characterized by X-Ray Diffraction, X-ray Photoelectron Spectroscopy, Surface area and Pore Size Distribution, Scanning Electron Microscopy, Thermogravimetric Analysis, H2-Temperature Programmed Reduction and Ammonia-Temperature Programmed Desorption. The results indicate that the surface area, interaction with the support, morphology of the catalyst and surface acidic sites exerted a significant influence on the activity.

Flower-like Al2O3-supported iron oxides as an efficient catalyst for oxidative dehydrogenation of ethlybenzene with CO2

An Al2O3 support with flower-like morphology was used as the support of iron oxide catalyst for oxidative dehydrogenation of ethylbenzene with CO2. A commercial Al2O3 was also used for comparison. These catalyst materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 physisorption, H2 temperature programmed reduction (H2-TPR), cyclic voltammetry (CV), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and CO2 temperature programmed desorption (CO2-TPD) techniques. It was found that the concentration of O2− anions on the flower-like Al2O3 surface was significantly higher than that on the commercial Al2O3 surface, leading to large amounts of highly dispersed FexOy clusters on flower-like Al2O3 in contrast with large Fe2O3 particles mainly present on commercial Al2O3. A possible formation mechanism of the small FexOy clusters was proposed. The catalytic results indicated that the highly dispersed FexOy clusters possessed an excellent catalytic activity, since the ethylbenzene conversion over Fe2O3/flower-like Al2O3 was 1.6 times higher than that over Fe2O3/commercial Al2O3. This work provides a new route for designing supported iron catalysts that are widely used, and flower-like Al2O3 is expected to be a promising catalyst support for oxidative dehydrogenation of ethylbenzene with CO2.

Nanoflake-assembled Al2O3-supported CeO2-ZrO2 as an efficient catalyst for oxidative dehydrogenation of ethylbenzene with CO2

An Al2O3 material assembled by nanoflakes was used to prepare supported CeO2-ZrO2 catalyst via a deposition-precipitation method for oxidative dehydrogenation of ethylbenzene with CO2. Both unsupported and commercial Al2O3-supported CeO2-ZrO2 were prepared for comparison. It was found that the CeO2-ZrO2/nanoflake-assembled Al2O3 catalyst exhibited the best catalytic activity. The characterization results revealed that the slit-shape pores existing in nanoflake-assembled Al2O3 were responsible for the small particle size and high Ce/Zr surface ratio of supported CeO2-ZrO2 species. The dispersion of Ce1-xZrx(OH)4 precursors onto Al2O3 support surface during the deposition-precipitation process was proposed. The high dispersion and large numbers of surface oxygen vacancies of the CeO2-ZrO2 species on nanoflake-assembled Al2O3 contributed to the excellent catalytic performance in oxidative dehydrogenation of ethylbenzene with CO2. This kind of special Al2O3 is expected to be a promising support for preparing highly dispersed metal/metal oxide catalysts.

The advantage of ceria loading over V2O5/Al2O3 catalyst for vapor phase oxidative dehydrogenation of ethylbenzene to styrene using CO2 as a soft oxidant

The present work highlights the influence of ceria over vanadia/alumina for the oxidative dehydrogenation of ethylbenzene to styrene with CO2 as a soft oxidant. Various weight loadings (0, 3, 5, and 7 %) of ceria were incorporated into 10wt % V2O5/Al2O3 catalyst by wet impregnation process. Structural and textural characterizations of the catalysts were performed by means of powder X-ray diffraction, temperature-programmed reduction, temperature-programmed desorption of CO2, N2 adsorption–desorption analysis. Over 10 % V2O5/Al2O3, ethylbenzene conversion decreases from 62 to 49 % in a spam of 12 h. Ceria-incorporated catalyst (3 % CeO2/10 % V2O5/Al2O3) showed steady ethylbenzene conversion of nearly 65 % with a styrene selectivity of 96 % for a period of 12 h time. Improved catalytic activity observed on this catalyst can be attributed to the increase in the number of redox/active VOx species after incorporation of ceria. The redox properties of 3 % CeO2/10 % V2O5/Al2O3 and 10wt % V2O5/Al2O3 were examined by reaction of CO2 in pulses on to the partially reduced catalysts. The spent catalysts were characterized using XRD & TPR analysis to assess the reason for the deactivation of catalysts.

The enhancement of the catalytic performance of CrOx/Al2O3 catalysts for ethylbenzene dehydrogenation through tailored coke deposition

The origin and role of coke in ethylbenzene dehydrogenation over CrOx/Al2O3 has been studied. Ethylene is the main coke precursor; however coke from aromatic species is beneficial for styrene production.

Mesoporous xEr2O3·CoTiO3 composite oxide catalysts for low temperature dehydrogenation of ethylbenzene to styrene using CO2 as a soft oxidant

Mesoporous xEr2O3·CoTiO3 composites tested as a new type of catalyst for the oxidative dehydrogenation of ethylbenzene to styrene.

Optimum ratio of K2O to CeO2 in a wet-chemical method prepared catalysts for ethylbenzene dehydrogenation

Fe2O3–K2O–CeO2 catalysts with various ratios of K2O to CeO2 were prepared by the wet-chemical method. Their phase compositions, reducibility, valence states of elements and catalytic activities for ethylbenzene dehydrogenation were studied. The results demonstrated that when the weight ratio of K2O: CeO2 was 1.40, the catalyst had highest ethylbenzene conversion and styrene selectivity, which were attributed to the optimization of active phase content and electron transfer ability, etc. Further, higher CeO2 content not only enhanced styrene selectivity, but also prolonged the life cycle of catalysts.

The effects of morphology of cerium oxide catalysts for dehydrogenation of ethylbenzene to styrene

Ethylbenzene (EB) dehydrogenation in presence and absence of CO2 was investigated over CeO2 catalysts of distinct morphologies: cubes, rods and particles. The presence of CO2 resulted in prolonged catalyst activity and higher initial benzene selectivity compared to the absence of CO2. However, CO2 had no effect on the catalytic activity of ceria catalysts after stabilization. CeO2 cubes exhibited about two times higher activity per m2 compared to rods and particles, independent of presence or absence of CO2. Product distribution and Raman spectroscopy characterization of the spent catalysts suggested that superior catalytic activity of CeO2 cubes originates from the enhanced amount of lattice oxygen species that can be extracted by EB on the (100) crystal planes of ceria cubes, contrary to highly intrinsically stable (111) crystal planes predominantly exposed on rods and particles. We suggest that these O species are consumed in nonselective EB conversion pathways, generating partly-reduced surface sites, which are suggested to act as active sites for selective styrene formation.

Macroscopic nanodiamonds/β-SiC composite as metal-free catalysts for steam-free dehydrogenation of ethylbenzene to styrene

Nanodiamonds (NDs), a typical sp3-like carbon, have an average size less than 10nm with a large surface-to-volume ratio, which contributes to a high surface activity than other types of carbon. However, the powder form of the material renders it difficult to use in large-scale applications, especially in the field of heterogeneous catalysis in which high pressure drop across the catalytic bed and difficulty of handling powder are the main drawbacks. In this work, a new hybrid structure consisting of nanodiamond-decorated porous β-SiC foam matrix has been investigated as a metal-free catalyst in the steam-free dehydrogenation of ethylbenzene into styrene. The hybrid foam catalyst displays an excellent catalytic specific rate and stability in the direct catalytic dehydrogenation of ethylbenzene (EB) to styrene. The dehydrogenation activity (12.87mmolST/gND/h, moles styrene products per gram of nanodiamonds per hour) is about 10 times higher with respect to the commercial K-Fe-based catalyst and 3.8 times higher than that of powder-form NDs without losing styrene selectivity (SST=98%) and thus demonstrates the high catalytic efficiency of this metal-free catalyst for the studied reaction. This study shows a new development towards the practical use of such structured composite catalysts in the field of catalysis where high performance and stable catalyst are still under scrutiny.

Effect of preparation methods on the catalytic properties of Fe2O3/Al2O3-ZrO2 for ethylbenzene dehydrogenation

To improve the catalytic performance of iron-based catalysts for dehydrogenation of ethylbenzene with CO2, two samples, F4AZ10-imp and F4AZ10-pre, with the same contents of Fe, Al and Zr were prepared by the impregnation method and one-step precipitation method. The catalytic performance of these two samples for the dehydrogenation of ethylbenzene with CO2 was tested at 550°C. Both the samples are characterized with XRD, NH3-TPD, CO2-TPD and H2-TPR techniques and N2 sorption experiment. It was shown that the preparation methods had a great effect on the catalytic performance. Compared to the impregnation method, the one-step precipitation method gave a more active catalyst because of a higher dispersion of active Fe2O3 on the catalyst surface and a stronger activation of CO2.

Activity studies of vanadium, iron, carbon and mixed oxides based catalysts for the oxidative dehydrogenation of ethylbenzene to styrene: a review

The activity of V, Fe and carbon based catalysts depends on V5+, Fe3+and active carbon sites. The mixed oxide catalysts show superior activity than individual oxides.