Oxidative Dehydrogenation of Ethylbenzene with Carbon Dioxide over Metal-Doped Titanium Oxides

Various metal-doped (Fe, V, Zr, Mg) titanium oxides were prepared by an acid-catalyzed sol–gel method and their properties as catalysts were investigated for oxidative dehydrogenation of ethylbenzene in the presence of carbon dioxide. The characterization techniques, XRD, BET, TGA were employed to analyze the features of catalyst. Fe₃Ti catalyst was found to be quite effective among the catalysts tested at 823 K, 39.8% ethylbenzene conversion and 98% styrene selectivity were acquired.     

Performance of metal‐oxide‐promoted LiCl/sulfated‐zirconia catalysts in the ethane oxidative dehydrogenation into ethene

The effects of some transition‐ and lanthanide‐metal oxides in LiCl/sulfated‐zirconia (SZ) catalysts on catalytic behavior in the oxidative dehydrogenation of ethane were investigated. It is found that modification of LiCl/SZ by metal oxides significantly improves the catalytic activity and ethene yield. Among those additives, Ni and Nd oxides show the best promoting effect in terms of ethane conversion and ethene yield. 93% ethane conversion with 83% selectivity to ethene has been achieved over the Nd₂O₃–LiCl/SZ catalyst at 650°C. In addition, those oxide‐promoted LiCl/SZ catalysts are also found to exhibit a longer stability in catalytic performance. Metal‐oxide additives change the chemical structure and surface redox properties, which accounts for the enhancement of activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

VxTi复合氧化物催化剂用于二氧化碳气氛乙苯脱氢的研究

采用溶胶-凝胶法制备了以TiO₂为基体的VxTi复合氧化物催化剂,该催化剂用于乙苯二氧化碳低温氧化脱氢制苯乙烯反应。考察了活性组分含量和焙烧温度对催化剂活性的影响。结果表明,活性金属钒的添加有助于提高脱氢反应性能,但存在一适量值,摩尔分数超过5%,催化脱氢活性下降。通过XRD分析发现,不同焙烧温度制备的VxTi催化剂中TiO₂的晶相不同,随着温度的升高,TiO₂的晶相将由锐钛矿型转变为金红石晶相。TiO₂锐钛矿型晶相有利于苯乙烯选择性的提高,而金红石晶相则不利于催化剂的脱氢反应。     

Gas-phase oxydehydrogenation of ethylbenzene with nitrobenzene by hydrogen transfer catalyzed reaction to produce styrene and aniline

Gas-phase catalytic hydrogen transfer reaction between ethylbenzene and nitrobenzene, to produce styrene and aniline, has been carried out at 360–460°C on amorphous AlPO₄, SiO₂, Al₂O₃, and on a natural sepiolite, as well as on the corresponding 20 wt% supported nickel catalysts. The influence of Cu as a second metal was also studied. Reactions were also carried out without nitrobenzene, under nonoxidative conditions. Catalytic activity under oxidative conditions was always comparatively higher than in nonoxidative conditions. In both cases, styrene yield and selectivity values obtained with support materials directly used as catalysts were better than those obtained with the corresponding Ni or Ni–Cu supported metal catalysts, with the only exception of SiO₂. The best results were obtained when amorphous AlPO₄ was used as the catalyst. The catalytic activity obtained in both oxidative and nonoxidative conditions, was closely associated to acid–base properties of the catalysts studied. Furthermore, a very similar linear correlation between A and E _a known as “compensation effect” was obtained and a common dehydrogenation mechanism was considered for oxidative and nonoxidative conditions. However, without considering the catalyst, nitrobenzene plays an important role as hydrogen acceptor, not only shifting the ethylbenzene dehydrogenation equilibrium but also avoiding secondary reactions by lowering the level of available hydrogen, especially when supported metals are being used as catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Catalytic dehydrogenation of isobutane over ordered mesoporous Cr2O3–Al2O3 composite oxides

A series of ordered mesoporous Cr₂O₃–Al₂O₃ composite oxides synthesized via improved one-pot evaporation induced self-assembly strategy were investigated as the catalysts for catalytic dehydrogenation of isobutane. These mesoporous catalysts with good structural properties and thermal stability performed excellent catalytic properties. Besides, the effect of the ordered mesopore structure on improving catalytic properties was also studied. Compared with non-mesoporous catalyst, the current mesoporous catalyst could accommodate the gaseous reactant with more “accessible” active sites. Therefore, the present materials were considered as promising catalyst candidates for catalytic dehydrogenation of isobutane.     

n‐octane reforming over alumina‐supported Pt, Pt–Sn and Pt–W catalysts

Pt, Pt–Sn and Pt–W supported on γ‐Al₂O₃ were prepared and characterized by H₂ chemisorption, TEM, TPR, test reactions of n‐C₈ reforming (500°C), cyclohexane dehydrogenation (315°C) and n‐C₅ isomerization (500°C), and TPO of the used catalysts. Pt is completely reduced to Pt⁰, but only a small fraction of Sn and of W oxides are reduced to metal. The second element decreases the metallic properties of Pt (H₂ chemisorption and dehydrogenation activity) but increases dehydrocyclization and stability. In spite of the large decrease in dehydrogenation activity of Pt in the bimetallics, the metallic function is not the controlling function of the bifunctional mechanisms of dehydrocyclization. Pt–Sn/Al₂O₃ is the best catalyst with the highest acid to metallic functions ratio (due to its lower metallic activity) presenting a xylenes distribution different from the other catalysts. The acid function of Pt–Sn/Al₂O₃ is tuned in order to increase isomerization and cyclization and to decrease cracking, as compared to Pt and Pt–W. This revised version was published online in July 2006 with corrections to the Cover Date.     

Novel Perovskite-Type Oxide Catalysts for Dehydrogenation of Ethylbenzene to Styrene

We investigated novel LaMnOx perovskite-oxide (ABO₃) catalysts for effective catalytic dehydrogenation of ethylbenzene to produce styrene monomer. Comparison with industrial Fe–K catalyst, our La_0.8Ba_0.2Mn_0.6Fe_0.4O_3-δ catalyst showed higher activity. Results show that the A-site in perovskite-type oxides affected catalytic dehydrogenation activities and that the B-site affected stability of the activities.     

Influence of SBA-15 support on CeO2–ZrO2 catalyst for the dehydrogenation of ethylbenzene to styrene with CO2

Pure oxides of ceria (CeO₂) and zirconia (ZrO₂) were prepared by precipitation method and a catalyst comprising of 25 mol% of CeO₂ and 75 mol% of ZrO₂ (25CZ) mixed metal oxide was prepared by co-precipitation method and also a catalyst with 25 wt% of 25CZ (25 mol% of CeO₂ and 75 mol% of ZrO₂) and 75 wt% SBA-15(25/25CZS) was prepared by precipitation–deposition method. Aqueous NH₃ solution was used as a hydrolyzing agent for all the precipitation reactions. These catalysts were characterized by X-ray diffraction and nitrogen adsorption–desorption techniques for the confirmation of SBA-15 structural intactness. All these catalysts were found to be effective for the oxidative dehydrogenation of ethylbenzene (ODHEB) to styrene in the presence of CO₂ and also it was observed that there was a sequential enhancement in the catalytic activity from individual oxides to mixed oxides followed by supported mixed oxide catalysts. Of the catalysts studied in this work, the supported 25/25CZS catalyst exhibited the superior activity, which was about 10–20 times higher than the activity of bulk single oxides in terms of turn over frequency.     

Hydrotalcite-like compounds as precursors for mixed oxides catalysts in the oxydehydrogenation of ethylbenzene

The dehydrogenation of ethylbenzene to styrene is a very important industrial reaction due to the multiple applications of styrene as a monomer for synthetic polymers. Reported catalysts that are active and selective in the oxidative dehydrogenation of alkylaromatics include supported metals through both main group and transition metal oxides to polymers and activated carbons. However, most of these catalysts are acid oxides, inducing the formation of an active coke layer that is the actual catalyst. We report in this work the use of various hydrotalcite-like compounds as precursors for the preparation of mixed oxides which are then used as basic catalysts in the oxydehydrogenation reaction. A series of materials has been synthesised by coprecipitation at constant pH and fully characterised by usual analytical techniques. The catalytic screening tests were carried out in a fixed bed quartz reactor at a temperature of 450°C and the liquid products were analysed off-line by gas chromatography. Some particularities of these materials are brought forward, like the absence of coking which however, does not seem to affect the activity, thus suggesting a different mechanism; the results show high selectivity in styrene, while the activity of the catalysts needs some improvements. For example, with vanadium containing oxides a selectivity of 98% in styrene is achieved with a conversion of 38% in ethylbenzene.     

Simultaneous dehydrogenation and isomerization of η‐butane to isobutene over ZSM‐22 and zinc‐modified ZSM‐5 zeolites

Direct formation of isobutene from η‐butane was investigated over a zinc‐impregnated potassium‐ion‐exchanged ZSM‐5 dehydrogenation catalyst and an acidic shape‐selective ZSM‐22 skeletal isomerization catalyst. The experiments were performed in a fixed‐bed microreactor system operating at near‐atmospheric pressure. High selectivity to η‐butene was obtained over the zinc‐impregnated potassium‐ion‐exchanged ZSM‐5 dehydrogenation catalysts. The yield of isobutene increased after adding the acidic ZSM‐22 skeletal isomerization catalyst, although the selectivity to butene isomers slightly decreased because the skeletal isomerization of η‐butene was competing with other acid‐catalyzed reactions such as cracking and aromatization. This revised version was published online in July 2006 with corrections to the Cover Date.     

High activity and stability of the Rh‐free Co‐based ex‐hydrotalcite containing Pd in the catalytic decomposition of N2O

The catalytic decomposition of nitrous oxide to nitrogen and oxygen has been studied over calcined hydrotalcite‐like compounds containing different combinations of bivalent (Co, Pd, Mg) and trivalent (Al, La, Rh) cations with carbonate as interlayer anion. The precursors were prepared by co‐precipitation under low supersaturation conditions and characterized by XRD and TG/DSC. The mixed oxides derived after calcination at 723 K were characterized by XRD, N₂ adsorption at 77 K, and XRF. The presence of Rh, La, or Pd in the Co‐based HTlc's improves considerably the catalytic activity. Co–Rh,Al‐HTlc (Co/Rh/Al==3/0.02/1) proved to be a very active catalyst, although the presence of the noble metal Pd in this catalyst ex‐Co,Pd–La,Al‐HT (Co/Pd/La/Al=3/1/1/1) produces a similar catalytic activity to that of Rh‐containing catalyst, both in a N₂O‐containing stream and in one containing also SO₂ and O₂, but with a better performance in stability tests. PdO phase has been identified by XRD as being responsible for the considerable improvement in the activity. The presence of Mg as spinel structure exerts a stabilizing effect in the more active catalysts when mixtures of SO₂ and O₂ are considered. This revised version was published online in July 2006 with corrections to the Cover Date.     

Nonoxidative dehydrogenation and aromatization of methane over W/HZSM‐5‐based catalysts

Highly active and heat‐resisting W/HZSM‐5‐based catalysts for nonoxidative dehydro‐aromatization of methane (DHAM) have been developed and studied. It was found from the experiments that the W−H₂SO₄/HZSM−5 catalyst prepared from a H₂SO₄‐acidified solution of ammonium tungstate (with a pH value at 2–3) displayed rather high DHAM activity at 973–1023 K, whereas the W/HZSM‐5 catalyst prepared from an alkaline or neutral solution of (NH₄)₂WO₄ showed very little DHAM activity at the same temperatures. Laser Raman spectra provided evidence for existence of (WO₆)^n- groups constructing polytungstate ions in the acidified solution of ammonium tungstate. The H₂‐TPR results showed that the reduction of precursor of the 3% W–H₂SO₄/HZSM‐5 catalyst may occur at temperatures below 900 K, producing W species with mixed valence states, W⁵⁺ and W⁴⁺, whereas the reduction of the 3% W/HZSM‐5 occurred mainly at temperatures above 1023 K, producing only one type of dominant W species, W⁵⁺. The results seem to imply that the observed high DHAM activity on the W–H₂SO₄/HZSM‐5 catalyst was closely correlated with (WO₆)^n- groups with octahedral coordination as the precursor of catalytically active species. Incorporation of Zn (or La) into the W–H₂SO₄/HZSM‐5 catalyst has been found to pronouncedly improve the activity and stability of the catalyst for DHAM reaction. Over a 2.5% W–1.5% Zn–H₂SO₄/HZSM‐5 catalyst and under reaction conditions of 1123 K, 0.1 MPa, and GHSV=1500 ml/(h g−cat.), methane conversion (XCH₄) reached 23% with the selectivity to benzene at ∼96% and an amount of coke for 3 h of operation at 0.02% of the catalyst weight used. This revised version was published online in July 2006 with corrections to the Cover Date.     

Screening of amorphous metal–phosphate catalysts for the oxidative dehydrogenation of ethylbenzene to styrene

The gas-phase oxidative dehydrogenation of ethylbenzene to styrene was carried out by using as catalyst a series of metal phosphates (Al, Fe, Ni, Ca and Mn) and stoichiometric (Al/Fe = Al/Ca = 1) mixed systems: FeAl(PO₄)₂ and Ca₃Al₃(PO₄)₅, that were prepared by an ammonia gelation method. Their amorphous character was determined through several physical methods: nitrogen adsorption, DRIFT and XRD patterns. These results were compared to those obtained with 24 commercial inorganic solids (several metal oxides, sulfates and phosphates). Reactions were also carried out without oxygen, under non-oxidative conditions, where the catalytic activity was always appreciably lower than under oxidative conditions. Experimental results indicated that the oxidative gas-phase dehydrogenation of ethylbenzene to styrene could be related to the total number of acid and basic sites of catalysts, so that this reaction probably needs selected acid–basic pairs for coke formation, where the oxidative dehydrogenation process is developed.

The main practical conclusion of the catalyst screening was that the best results were obtained with the synthesized amorphous AlPO₄, where 43% ethylbenzene conversion and 99.7% styrene selectivity were achieved. A very reduced number of commercial inorganic solids like Al₂(SO₄)₃, Cr₂(SO₄)₃, Fe₂(SO₄)₃, NiSO₄, Al₂O₃ and Fe₂O₃ were also able to obtain an acceptable catalytic behavior, with conversions ranging between 18 and 23% and selectivity in the 95–100% range. Among the other synthesized solids, Ni₃(PO₄)₂-A-450 was the only metal phosphate exhibiting results in such a range. All the other catalysts studied were rather inactive and/or selective. Additional experiments carried out at longer times on stream (3.5 h) and longer contact times (W/F 0.254 and 0.654) confirmed the superior catalytic behavior of amorphous AlPO₄. Consequently, this solid could be a good candidate for application as a catalyst in the industrial oxydehydrogenation of ethylbenzene to styrene.     

New water‐tolerant supported molten indium catalyst for the selective catalytic reduction of nitric oxide by ethanol

The catalytic activity and selectivity of indium supported on controlled pore glass (In‐CPG‐SMMC) were investigated and compared with those of H‐ZSM‐5 and In/γ‐Al₂O₃ for the selective catalytic reduction of nitric oxide by ethanol under net oxidizing conditions, in the absence and presence of water vapors. Even though the support of In‐CPG‐SMMC is almost pure silica (94–99% SiO₂, 1–6% B₂O₃, 0.05–0.3% Na₂O), the activity of this catalyst was found to be comparable with that of the conventional catalysts. The presence of steam in the feed enhanced catalyst activity over the entire temperature range studied. At temperatures above 500°C the ability of H‐ZSM‐5 and In/γ‐Al₂O₃ to reduce NO to N₂ was surpassed by the supported indium catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

Selective oxidation of propane on MgO/γ‐Al2O3‐supported molybdenum catalyst: influence of promoters

The influence of promoters, potassium and samarium, on molybdenum supported over MgO–γ‐Al₂O₃ catalyst has been investigated in the oxidative dehydrogenation of propane. The acidities of catalysts were determined by temperature‐programmed desorption of NH₃ and by decomposition of 2‐propanol. The K‐promoted catalyst showed the lower acidity followed by the Sm, whereas the unpromoted sample showed the highest acidity. The higher the acid character of the catalyst, the lower the selectivity to propene. Redox properties determined from EPR spectra change with the addition of the promoter. A parallelism between Mo⁶⁺ reducibility and catalytic activity was found. This revised version was published online in July 2006 with corrections to the Cover Date.     

Behaviors of V-doped Titanium Mixed Oxides in the Catalytic Dehydrogenation of Ethylbenzene

By an acid-catalyzed sol–gel method the V-doped titanium oxides (VxDTOs, x _mol% of V/(V + Ti) molar ratio) were prepared and systematically characterized by the techniques of XRD, TEM, H₂-TPR, XPS and TGA, indicating that most of the vanadium in the catalyst was highly dispersed on the surface of and in the bulk phase of TiO₂. The effect of V⁵⁺ doping level (1–9 mol% V⁵⁺) on the surface of TiO₂ and VxDTOs catalytic performance was analyzed. From the catalytic activity evaluation of VxDTOs in the dehydrogenation of ethylbenzene to styrene in the presence of CO₂ (CO₂-EBDH) at 450 °C, V6DTOs catalyst had the highest activity. Its higher activity in CO₂ than in N₂ clearly shows that CO₂ markedly enhanced the dehydrogenation of ethylbenzene. The activity of V6DTOs catalyst could almost be resumed after regeneration by air while partly resumed after regeneration by CO₂.     

Novel Fe‐based complex oxide catalysts for hydroxylation of phenol

Novel catalysts for the hydroxylation of phenol, Fe–Si–O, Fe–Mg–O and Fe–Mg–Si–O complex oxides, have been synthesized by a coprecipitation method. X‐ray diffraction studies show that MgFe₂O₄ crystallites with spinel structure are formed in Fe–Mg–Si–O and Fe–Mg–O complex oxides and the crystallite size of the metal oxide or complex oxide is reduced after addition of Si. In the hydroxylation of phenol with hydrogen peroxide, Fe‐based complex oxides exhibit high activities after a short induction period. The phenol conversion is improved when silicon is introduced into the Fe‐based complex oxides, and formation of MgFe₂O₄ crystals with spinel structure in the catalysts increases the diphenol selectivity. The addition of a little acetic acid to the reaction liquid can shorten the induction period effectively. Under the same reaction conditions, phenol conversion and diphenol selectivity over the Fe–Mg–Si–O catalyst are close to those over TS‐1, and furthermore, the reaction time is more than ten times shorter as compared to TS‐1. The reaction mechanism of the hydroxylation of phenol on the catalysts has been studied, and a free‐radical mechanism initiated by the formation of phenoxy free radicals is suggested. This revised version was published online in August 2006 with corrections to the Cover Date.     

High Performance of Fe–K Oxide Catalysts for Dehydrogenation of Ethylbenzene to Styrene with an aid of ppm-order Pd

Styrene is manufactured industrially through catalytic dehydrogenation of ethylbenzene on Fe–K oxide-based catalysts. It was invented by Süd-Chemie Group that the activity of the industrial ethylbenzene dehydrogenation catalysts (Styromax) based on the oxides of Fe and K is highly promoted by the addition of small amount (hundreds ppm-order) of precious metals such as Pd. The present work is intended to elucidate the role of Pd on the Fe–K catalyst empirically by use of a periodical pulse technique from a mechanistic point of view. The oxidative dehydrogenation was faster than the simple dehydrogenation, and it proceeded by consuming the surface lattice oxygen in the catalyst. The lattice oxygen was subsequently supplied from steam. Palladium added to the Fe–K oxide catalysts was found to enhance the rate of regeneration (supplying) of the lattice oxygen, although it hardly changed the rate of dehydrogenation of ethylbenzene or consumption of surface lattice O^2? anions. This study demonstrated that steam works not only as a diluent but also as a reactant to form hydrogen and lattice oxygen.     

Contribution of components in Ce–Ti–Cr–Ox catalysts for selective propane-combustion in the presence of CO

Gas-phase combustion of hydrocarbons and CO in exhaust gases are normally performed competitively by supported noble metals. With the help of high-throughput technologies complex mixed oxides, such as the amorphous porous Ce₂₀Ti₅₀Cr₃₀O_x have been discovered, which selectively convert propane in the presence of excess of carbon monoxide. These findings are of fundamental importance for heterogeneous catalysis and may have implications on the future development of novel exhaust gas catalysts. In this study the effect of the various elements of the mixed oxide catalysts on activity and selectivity has been investigated and interpreted. The results of attempts to further improve the catalysts by additional doping are presented.     

Modification of the alumina‐supported Mo‐based hydrodesulfurization catalysts by tungsten

The incorporation effect of tungsten as an activity‐promotional modifier into the Ni‐promoted Mo/γ‐Al₂O₃ catalyst was studied. Series of W‐incorporated catalysts with different content of tungsten were prepared by changing the impregnation order of nickel and tungsten onto a base Mo/γ‐Al₂O₃. Catalytic activities were measured from the atmospheric reactions of thiophene hydrodesulfurization (HDS) and ethylene hydrogenation (HYD). The HDS and HYD activities of the WMo/γ‐Al₂O₃ catalysts (WM series) initially increased and subsequently decreased with increasing content of tungsten as compared with those of their base Mo/γ‐Al₂O₃. The maximal activity promotion occurred at the W/(W + Mo) atomic ratio 0.025. For the Ni‐promoted Mo/γ‐Al₂O₃ catalysts, the effect of W incorporation was greatly dependent on the impregnation order of tungsten. The catalysts prepared by impregnating Ni onto the WMo/γ‐Al₂O₃ catalysts showed the same trend of activity promotion as for the WM series, while those by impregnating W onto a NiMo/γ‐Al₂O₃ catalyst resulted in lower activities than their base NiMo/γ‐Al₂O₃ catalyst. To characterize the catalysts, temperature‐programmed reduction and low‐temperature oxygen chemisorption were conducted. The effects of W incorporation on the NiMo‐based catalysts were discussed in reference to those on the CoMo‐based catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.