Skeletal Isomerization Reaction Research Articles

Catalytic Activity of Bulk WO3 for the Reforming of Hexanes and Hexenes

The effects of the reduction process, under a hydrogen flow, of a bulk WO3 tungsten oxide powder on the reactivity at 350°C of 2-methylpentane (2MP) and 4-methyl-1-pentene (4M1Pene) were investigated. The best experimental conditions to obtain a stable state exhibiting interesting skeletal isomerization properties were determined. Catalyst characterization by XPS, XRD, TPR, and surface area measurements allowed us to propose some intermediates phases and the corresponding nature of active sites formed during the reduction process of the WO3 tungsten oxide. Correlations between these catalyst characterizations and the catalytic properties show that the WO3 phase is initially inactive for both alkanes and alkenes reactions. When the W20O58 phase is formed, which corresponds to the first reduction step of WO3, the surface becomes acidic in character, leading to the skeletal isomerization of alkenes by a monofunctional acidic mechanism. The reduction of the W20O58 tungsten oxide occurs in two parallel ways, to WO2 and W3O. When this reduction is performed at moderate temperature (350°C), a stable surface composed of these two phases (WO2 and W3O) is obtained. This surface is active and very selective for the skeletal isomerization reactions of alkanes and alkenes. This catalytic behavior has been interpreted with the aid of two mechanisms: a bifunctional one on the WO2 phase and a monofunctional metallacycle one on the W3O phase.

Thermodynamic and Kinetic Reaction Regimes in the Isomerization of 1-Pentene over ZSM-5 Catalysts

Two interesting observations have been made during isomerization of 1-pentene over H-ZSM-5 catalyst. (a) It is possible to switch between two thermodynamically controlled regimes by changing the reaction temperature. At low temperature, only double bond isomerization is occurring whereas the total isomerization equilibrium involving both n- and isopentenes is attained at high temperature. The temperature range of the kinetically controlled regime between these two equilibrium states is narrow. The transition is accompanied by a sharp increase in the 1-pentene conversion from 90% to almost 100%. The skeletal isomerization reactions are sufficiently fast to maintain an equilibrium distribution although pentenes are continuously withdrawn for the formation of dimerization and cracking products. Changing the space velocity or the feed partial pressure did not result in a stretching or squeezing but rather in a temperature shift of the kinetic regime. (b) Increasing 1-pentene partial pressures favor skeletal isomerization over dimerization and cracking reactions. This result, unexpected at first sight, suggests that reactions yielding isopentenes exhibit a higher reaction order in n-pentenes than the competing reactions. We have also investigated the dehydroisomerization of n-pentane over a single fixed bed of bifunctional Pt/H-ZSM-5 and over stacked beds of Pt/γ-Al2O3 and H-ZSM-5. The results confirm that very low partial pressures of n-pentenes, here generated in situ, have a negative effect on the yields of isopentenes.

Recent applications in catalysis of surface organometallic chemistry

This paper reviews recent applications of well-defined silica-supported hydrides of the group 4 and 5 transition metals in the field of carbon—carbon and carbon—hydrogen bonds activation of alkanes. The synthesis and characterization of the zirconium hydride is presented. The monohydride (SiO) 3Zr–H is obtained by hydrogen treatment at ca. 150°C of the well-defined surface species Si–O–ZrNp 3 (Np=CH 2C(CH 3) 3). This surface complex is formally an 8 electron species and is consequently very electrophilic. Similarly, hafnium and titanium hydrides are obtained by treatment under hydrogen of Si–O–MNp 3 (M=Hf,Ti). In the case of titanium the reaction is not quantitative in the sense that a non-negligible amount of titanium(III) is formed. The tantalum hydride (SiO) 2Ta–H is obtained by hydrogen treatment at ca. 150°C of (Si–O) x Ta(CHC(CH 3) 3)(CH 2C(CH 3) 3) 3− x ( x=1,2), prepared by reaction of Ta(CHC(CH 3) 3)(CH 2C(CH 3) 3) 3 with the hydroxyl groups of silica. Examples of applications of these hydrides in the field of the activation of alkanes at moderate temperatures are then given. All these surface hydrides can achieve the hydrogenolysis of alkanes at low temperature. When the titanium hydride is used, a simultaneous reaction of skeletal isomerization occurs. In all cases, the mechanism of C–C bond cleavage passes through an elementary step of β-alkyl transfer. The mechanism of hydroisomerization observed with the titanium hydride passes also by an elementary step of β-alkyl transfer but, in this case, the β-H elimination-olefin reinsertion occurs quite rapidly so that a skeletal isomerization also occurs. The zirconium hydride can also catalyze under olefin pressure the olefin polymerization and under hydrogen pressure the polyolefin hydrogenolysis. Here the equilibrium between the olefin insertion into a metal alkyl and the β-alkyl transfer is shown to occur with the same catalyst in agreement with the concept of microreversibility. A new catalytic reaction called `alkane metathesis' has been discovered with the tantalum hydride. By this reaction, alkanes are catalytically transformed into higher and lower alkanes. The mechanism by which this reaction occurs is not fully understood. The products distribution, especially with labeled alkanes, is explained by a concerted mechanism by which a Ta–C bond and a C–C bond of the alkane can be cleaved and reformed simultaneously via a kind of four centered σ-bond metathesis which has no precedent in classical organometallic chemistry.

SKELETAL ISOMERIZATION OF LINEAR BUTENES: THE CATALYST AND THE KINETIC MODELING

Silico-Alumino-Phosphate(SAPO-11) is prepared by using the available Chinese chemical reagents. The prepared catalyst is characterized by both XRD and TPD. The synthesized catalyst is then used to study the chemical kinetics of skeletal isomerization reaction of linear butenes to iso-butene. A practical approach is used here, since we conducted the experiments on an actual butenes fractions produced in a commercial refinery. Results show that the yield of iso-butene increases with temperature increase to reach the equilibrium ratio. The transformation of linear butenes into iso-butene can be represented by a first order reversible reaction model. Reaction rate constants for both the forward and backward reactions as well as activation energies were calculated.

Structure, Texture, Surface Acidity, and Catalytic Activity of AlPO4–ZrO2(5–50 wt% ZrO2) Catalysts Prepared by a Sol–Gel Procedure

A series of AlPO4–ZrO2(APZr) systems with various zirconia loadings (5–50 wt%) were prepared by sol–gel deposition of zirconia onto AlPO4and characterized by TGA/DTA, XRD, DRIFT, laser-Raman spectroscopy, SEM–EDX, XPS,27Al and31P MAS NMR, and nitrogen adsorption. Surface acid characterization was carried out through pyridine chemisorption and cyclohexene skeletal isomerization reaction. Addition of ZrO2onto AlPO4prevented the formation of monoclinic zirconia and increased the temperature of crystallization into the tetragonal form, so that APZr systems were still amorphous after calcination at 1073 K. This higher thermal stability was responsible for its large surface area as well as pore volume in relation to pure components. SEM showed nonhomogeneous distribution in morphology, texture, and particle size. Also, EDX and XPS indicated that the surface concentration ratios were practically the same as the expected values of stoichiometric composition. Moreover,27Al and31P MAS NMR showed that the local structure of the AlPO4support did not change with zirconia. Good correlations were obtained between catalytic activities in cyclohexene skeletal isomerization and surface acid densities of AlPO4–ZrO2systems (as determined by pyridine chemisorption at 373 and 573 K).

Preparation and properties of new acid catalysts obtained by grafting alkoxides and derivatives on the most common supports. Part II: Grafting zirconium and silicon alkoxides on γ-alumina

In this work, the preparation and properties of the acid catalysts obtained by grafting zirconium and silicon alkoxides and zirconium-sulfated alkoxide on γ-alumina were studied. The catalysts prepared were examined with electrochemical techniques to evaluate the hydroxyl density, the ZPC (zero point charge) and the intrinsic surface ionization constants. The catalysts were also submitted to TPD (temperature-programmed desorption) analysis using different organic bases to determine the distribution of acid sites. Some samples were also submitted to microcalorimetric analysis using pyridine as probe molecule, and to XPS measurements. Finally, the catalysts prepared were tested in three different reactions: methanol dehydration, n-hexane isomerization and cracking and skeletal isomerization of 1-butene to isobutene. The most relevant results obtained were: (i) grafted zirconia strongly promoted methanol dehydration up to an upper limit of grafting; (ii) grafted zirconium sulfated alkoxide showed a moderate activity, but good selectivity in n-hexane isomerization; (iii) silicated aluminas (prepared in the absence of solvents) showed high activity, selectivity and stability in the 1-butene skeletal isomerization reaction.

Acidity and catalytic activity of AlPO 4–B 2O 3 and Al 2O 3–B 2O 3 (5–30 wt% B 2O 3) systems prepared by impregnation

The surface acidic properties of a series of AlPO 4–B 2O 3 (APB) and Al 2O 3–B 2O 3 (AlB) systems, prepared by supporting B 2O 3 from boric acid solutions using an incipient wetness method and containing different boria loading (5–30 wt%), have been studied by adsorption (at 573 K) of pyridine (PY) and 2,6-dimethylpyridine (DMPY) using a pulse-chromatographic method. Acidity measurements show that Lewis and Brønsted sites exist in the APB and AlB systems at all boria loadings. Moreover, the concentration of Brønsted acid sites increases with boria content. This increased surface Brønsted acidity is responsible for their improved performance in the cyclohexene skeletal isomerization (SI) reaction, to 1- and 3-methylcyclopentenes (MCPE), especially at high boria loadings. Comparison of the activity and acidity measurements points to a correlation between the activity and the number of Brønsted acid sites on the APB and AlB catalysts. Moreover, at high boron contents, AlB catalysts are more acidic and hence more active than APB catalysts. Furthermore, the more acidic catalyst (AlB, with 30 wt% boria) also showed catalytic activity for the cyclohexene hydrogen transfer (HT) reaction leading to methylcyclopentane and cyclohexane.

Deep Desulfurization of Refractory Sulfur Species in Gas Oil Using Skeletal Isomerization Reaction over Zeolite Catalyst (Part1) Quantum Chemical Analysis of Desulfurization Reactivity of Alkyldibenzothiophenes.

軽油中に含有される難脱硫性硫黄化合物4,6-ジメチルジベンゾチオフェン (4,6-DMDBT) をn-デカン中, 270°C, 水素圧2.5MPaでNi担持Y-ゼオライト (Ni-HYゼオライト) を用いてメチル基の異性化反応を行い, 引き続き中間体であるアルキルジベンゾチオフェン類 (Cx-DBT) を同反応条件でCoMo/Al2O3で脱硫し, それらの脱硫反応速度を算出した。また, 反応中間体の構造を決定するため, 数種類のアルキルジベンゾチオフェン類を合成した。このようにして構造決定できたアルキルジベンゾチオフェン類の脱硫反応速度の実測値と, 分子軌道計算で得られた硫黄原子上の電子密度との関係を考察し, ゼオライトによるメチル基転位反応が難脱硫性硫黄化合物の脱硫反応性に及ぼす影響を調べた。Ni-HYゼオライトによる異性化反応で生成した側鎖炭素数1～4のアルキルジベンゾチオフェン類は, 脱硫反応速度の違いにより以下の4グループに分類できた: 4,6-DMDBT (グループI)<4-MDBT, 3,6-DMDBTおよび未同定のC3-DBT類 (グループII)<2,3,4-トリメチルジベンゾチオフェン, および未同定のC3-DBT類 (グループIII)<2,8-DMDBT, および未同定のC1-DBT (グループIV)。これらの化合物の硫黄原子の非結合電子軌道 (Sσn軌道; 分子平面方向) は脱硫反応速度と良い相関を示した。また, このSσn軌道の電子密度は, 4,6-ジメチルジベンゾチオフェンのメチル基が4-位あるいは4,6-位から移動するに伴って増加した。分子軌道の解析から, アルキルジベンゾチオフェンの硫黄原子が触媒活性点に最接近する場合, 硫黄原子のSσn軌道の断面積は4,6-DMDBTの11A2から, 4-MDBTで20A2, DBTで42A2まで増加した。これらの知見から, 4,6-ジメチルジベンゾチオフェンの脱硫反応におけるゼオライトの役割は, メチル基を転位させて硫黄原子近傍の立体障害を解消し, 硫黄原子と触媒活性サイトとの接近を容易にするとともに, 硫黄原子の脱硫活性サイトへの配位を促進することであると結論した。

Gas‐phase pinacol conversion on AlPO4, γ‐Al2O3 and SiO2 catalysts

Pinacol (2,3‐dimethyl‐2,3‐butanediol) conversion over AlPO4 (Al/P = 1) and γ‐Al2O3 catalysts proceeded in two parallel reaction pathways with formation of 2,3‐dimethyl‐1,3‐butadiene (by 1,2‐elimination) and 3,3‐dimethyl‐2‐butanone (by rearrangement), the latter being the main reaction product. The activity was in accordance with the surface acidity data as measured versus cyclohexene skeletal isomerization reaction. Thus, AlPO4 showed the highest activity (almost total conversion at 523 K). The 1,2‐elimination/rearrangement ratio depended on the type of catalyst used and diene formation increased with reaction temperature. Moreover, pinacolone was a reaction intermediate for diene production.

Catalytic Activity of ZSM-22 Zeolites in the Skeletal Isomerization Reaction of 1-Butene

Skeletal isomerization of 1-butene was carried out over ZSM-22 zeolites in a fixed-bed reactor system at near atmospheric pressure. The ZSM-22 zeolites obtained after the synthesis and subsequent calcination showed high activity in the transformation of 1-butene even without ion-exchange procedures. There was a great difference in the catalytic activity between catalysts calcined in a separate calcination oven and catalysts calcined using high flows in the reactor. When a selective isomerization catalyst was prepared using calcination in air, a high flow was required in order to efficiently remove steam and heat. Calcination without any possibility of steam formation could be performed in a flow of nitrogen or even in the reactant flow at temperatures as low as 400 °C. The effect of temperature, time on stream, weight hour space velocity (WHSV), and partial pressure of 1-butene on the skeletal isomerization reaction of 1-butene was studied over the prepared ZSM-22 catalysts. ZSM-22 demonstrated high activity in the skeletal isomerization of 1-butene at extremely high WHSV, even at temperatures as low as 400 °C.

Catalytic skeletal isomerization of linear butenes to isobutene

The increased demand for isobutene, used for the production of the octane-enhancer methyl tert-butyl ether, has generated tremendous interest in the catalytic conversion of the linear butenes to isobutene. In this review we survey the progress made since the late 1970s in implementing the catalytic skeletal isomerization reaction of these linear alkenes. Halogenated catalysts, especially those based on alumina, and prepared using a variety of compounds of fluorine, chlorine or bromine, have been shown to exhibit both high conversions and selectivities for the reaction, resulting in high yields of isobutene, when water is added to the feed stream. Elution of the halogen from the catalyst leads to the loss of catalytic activity and necessitates the continuous or discontinuous addition of the halogen compound. As a consequence, environmental and other considerations are most likely to weigh against the industrial usage of these catalysts. Another class of catalysts exhibiting high activities and selectivities, again in the presence of water, are the silicated aluminas. No information is, however, available on their long-term stability. Even alumina on its own displays high activity and selectivity, provided water is co-fed with the hydrocarbon stream. More recent results obtained over other types of catalysts such as zeolites and molecular sieves are also presented. Most promising are the results obtained with the zeolite ferrierite which gives high yields of the branched isomer in the absence of any other additive or diluent. The catalyst also appears to be fairly stable showing no decrease in the yield of isobutene after 14 days on-stream. The high yields of isobutene can be ascribed to the small channel diameters which prevent the extensive dimerization or oligomerization of the linear butenes or of the product isobutene. Plans for the first large-scale demonstration plant to produce isobutene from n-butenes using ferrierite as catalyst have already been announced in the United States.

Comparison between Zeolite β and γ-Al2O3 Supported Pt for Reforming Reactions

The performance of zeolite β supported platinum as a bifunctional catalyst for the reforming of n-hexane, methylcyclopentane, and cyclohexane, typical representatives of reforming feeds, has been examined under low severity conditions. A comparison under the same conditions with γ-Al2O3 supported Pt was carried Out. Both catalysts were impregnated with 0.5 wt% Pt. It is found that β-zeolite exhibits skeletal isomerization reactions to a higher extent than alumina, producing molecules with much higher octane number than the feed molecules. Bimolecular alkylation reactions that take place in the zeolite pores have as a result the formation of products with a carbon number larger than that of the feed molecule. The enlargement of the cyclic ring of methylcyclopentane leads at suitable temperatures to aromatics, a reaction of great importance for reforming. This behavior is a result of the relatively large acidity of the zeolite and of its pore structure that favors the proximity among different molecules increasing the probability for reaction. In contrast, Pt/Al2O3 stimulates dehydrogenation-hydrogenation and to a smaller extent skeletal isomerization. The β-zeolite is deactivated more easily than alumina due to the coke build up on the acidic sites. The characterization of the catalysts has been carried out using XRD, NH3-TPD, SEM, FTIR, BET, TEM, and H2-Chemisorption.

Cyclohexene skeletal isomerization activity of sepiolites modified with B3+ or Al3+ ions

The effect of sepiolite modification by B3+ and Al3+ ions on catalytic performance were investigated in cyclohexene skeletal isomerization (CSI) reaction. Catalytic activity to 1- and 3-methylcyclopentenes (1- and 3-MCP) fit the Bassett-Habgood kinetic model for first order processes. These ions induced strong acidity in the sepiolite and the best results (greater activity and selectivity to 1-MCP) were found when the sepiolite was exchanged with an BF3/methanol solution which contained 3 wt.% of the B3+ ion.

Catalytic behaviour of polycrystalline Pt3Ti in relation to strong metal-support interaction phenomenon

Skeletal isomerization reactions, of hexanes (2-methylpentane and methylcyclopentane) have been carried out in a flow reactor on a polycrystalline intermetallic Pt3Ti compound which has been powdered and treated under several oxidative and reductive conditions. The results are compared to those obtained with (i) TiO2 supported Pt catalysts in or not in the Strong Metal-Support Interaction (SMSI) state, (ii) a bulk Pt3Ti sample studied under static catalytic conditions in an U.H.V. apparatus, and (iii) bulk Pt surfaces. The powdered Pt3Ti compound shows a very unstable behaviour under hydrogen. Differences in behaviour between the bulk and the decomposed powdered Pt3Ti samples are observed, showing the important part played by Pt3Ti in SMSI phenomena. A model is then proposed.

Influence of catalyst pretreatment on n-hexane transformations on platinum black

The hydrogenolysis and skeletal isomerization reactions of n-hexane on Pt-black have been studied as a function of thermal pretreatments of catalysts in different atmospheres. The catalytic behavior of platinum, particularly concerning the selectivity towards the above reactions, was found to be very sensitive to the presence of hydrogen or helium in the atmosphere. The mechanistic implications of the results obtained are discussed.

Thermally induced degenerate skeletal rearrangement and isomerization reactions of cyclooctatetraenes

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTThermally induced degenerate skeletal rearrangement and isomerization reactions of cyclooctatetraenesLeo A. Paquette, Masayoshi Oku, William E. Heyd, and Robert H. MeisingerCite this: J. Am. Chem. Soc. 1974, 96, 18, 5815–5825Publication Date (Print):September 1, 1974Publication History Published online1 May 2002Published inissue 1 September 1974https://doi.org/10.1021/ja00825a020RIGHTS & PERMISSIONSArticle Views245Altmetric-Citations24LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Relation between catalytic properties and structure of metal films: II. Skeletal reactions of some C 6 alkanes

Skeletal isomerization, cyclization, and hydrogenolysis reactions of n-hexane and 2-methylpentane have been studied on various platinum and nickel film catalysts which were prepared under UHV conditions. The reactions were carried out in the presence of excess hydrogen in a static system at 273 °C, and conversions were restricted to <5%. The work particularly compared the catalytic behavior of ultrathin and thick films, where in the former the average metal density on the substrate was of the order of a monolayer. Nevertheless, these ultra-thin films actually consist of crystals <20 Å across, possibly together with monodispersed platinum atoms. The most detailed information was obtained from the reactions of n-hexane. On all nickel films hydrogenolysis (mainly to methane) was the dominant reaction, while on all platinum films the formation of C 6 isomerization and cyclization products was the more important. Nevertheless, with both metals the reaction over ultra-thin films had a much higher selectivity for the formation of C 6 products versus hydrogenolysis products than did the reaction over various thick films. Furthermore, ultrathin platinum films gave an increased proportion of cyclic C 6 products compared to thick films. The variation in catalytic selectivity, particularly with platinum, is interpreted on the following model: (1) the very small crystals of the ultra-thin films present an increased proportion of low-coordinated corner atoms, (ii) these corner atoms favor the formation of adsorbed carbocyclic reaction intermediates: these intermediates can lead to cyclic reaction products as well as providing a contributing path for isomerization, (iii) hydrogenolysis as well as isomerization by a CC bond shift mechanism are not favored by corner atoms, but require two or three adjacent platinum atoms on a crystal face; the triadsorbed intermediate which requires three platinum atoms is favored by a (111) face or facet, and itself favors isomerization by CC bond shift versus hydrogenolysis. Because the selectivities (versus hydrogenolysis) for isomerization by an adsorbed carbocyclic intermediate and by CC bond shift vary in different ways with the concentration of low coordinated corner platinum atoms, and because the relative importance of these two isomerization paths are influenced by hydrocarbon geometry, the overall dependence of selectivity on catalyst structure may, for a given hydrocarbon, be complex, and requires analysis in terms of the contributing mechanisms. Reactions with 2-methylpentane were so heavily in favor of the formation of methylcyclopentane that the dependence on films structure was not large. UHV ultra-thin films form a useful model system for studying the effects of particle size on catalyst selectivity without the complication due to possible surface contamination.