Oxidative Dehydrogenation Of N-octane Research Articles

Faujasite silicalites for oxidative dehydrogenation of n-octane: Influence of alkali metals, gallium, and boron on catalyst activity

• Faujasite silicalites made using sol-gel method and tested in ODH of n-octane. • Extraframework barium reduced the acid site density, hence also COx formation. • Framework B gave more selective to octene isomers compared to framework Ga. • Ga facilitated burning of coke on the surface of the catalyst, prolonging activity. The sol-gel method was used to synthesize faujasite type silicalites bearing gallium and boron in the framework. Barium and sodium were used as charge balancing cations since isomorphic substitution of Si 4+ by Ga 3+ or B 3+ results in a negative excess charge of the framework. The successful synthesis of this type of silicalites (GaBaY-S, BBaY-S, GaBBaY-S(IE), GaNaY-S) was confirmed using powder-XRD. SEM analysis showed that the morphology of the catalysts with respect to particle size depended on the framework metals and the charge balancing cation used. Framework Ga containing catalysts showed smaller particle size compared to B containing catalysts. Sodium also yielded a smaller particle-sized catalyst compared to barium. The catalysts were tested in the continuous flow oxidative dehydrogenation (ODH) of n -octane, and the catalytic results showed dependence on the active metal reducibility and acid-base character of the catalysts. At iso-conversion of 8 ± 1 %, the least acidic BBaY-S gave the highest selectivity to octenes (40 %) and the lowest selectivity to COx (28 %), and the most acidic GaNaY-S showed the opposite results with octenes at 17 % and COx at 56 %. The catalysts (BaBY-S and GaBBaY-S(IE) with least total acidity had the greatest quantity of strong acid sites which were attributed to Lewis acid sites, confirmed by the pyridine IR analysis. The GaNaY-S, with the highest total acidity, had the least strong acid sites.

Oxidative dehydrogenation of n-octane using Ba and Ga-modified faujasite type catalysts prepared by different methods

Ionic exchange and isomorphic substitution were used for the synthesis and modification of faujasite (zeolite and silicalite Y) catalysts. These two methods were used to synthesize 1 wt% extra-framework and framework gallium modified faujasite zeolites respectively, with a Si/M ratio of 2.6, where M = Al and/or Ga. For the silicalite catalyst, 3.5 wt% of Ga was used to maintain the Si/Ga ratio. A series of BaY, Ga-BaY(IS) and Ga-BaY(Sil) were prepared by isomorphic substitution, where (IS) and (Sil) represent Isomorphic Substitution and Silicalite respectively. Ga-BaY(IE) and BaGa-NaY(IE), were prepared by ionic exchange (IE) using the synthesised BaY and a commercial NaY for the latter. For all the prepared catalysts, barium was kept constant at about 4.2 wt% except for BaGa-NaY(IE), where Ba was 1.4 wt%. For the sodium containing catalysts, Na was 10.3 wt%. Pyridine IR and chemisorption techniques showed that the catalysts prepared had more Lewis acid sites compared to Bronsted acid sites. The Na containing zeolite had the highest total acidity compared to those containing only Ba. Aluminium free silicalite contained the least total acidity, which is consistent with previous studies. Catalytic data at iso-conversion (7 ± 1%) showed that the Ba incorporated by ionic exchange zeolites and the silicalite were more selective to octenes (36 and 33% respectively) and less selective to COx (35 and 33% respectively) compared to the isomorphically substituted Ba catalysts. Selectivity to cracked products was reduced when aluminium, responsible for the strong Bronsted acid sites, was eliminated from the synthesis.

Effects of Modifying Acidity and Reducibility on the Activity of NaY Zeolite in the Oxidative Dehydrogenation of n-Octane

Non-coking stable alkaline earth metal (M = Mg, Sr, and Ba) modified Ga-NaY catalysts were prepared by ionic-exchange and tested in oxidative dehydrogenation (ODH) of n-octane using air as the source of oxygen. The role of the alkaline earth metals in NaY was to poison the acid sites while enhancing the basic sites responsible for ODH. The exception was the calcium modified NaY, which was more acidic than the parent NaY, coking and unstable under the ODH conditions used in this study. The role of gallium was to enhance the dehydrogenation pathway and improve the stability of NaY. The sequence of increasing selectivity to octenes followed the order: CaGa-NaY < Ga-NaY< MgGa-NaY < SrGa-NaY < BaGa-NaY. The highest octene selectivity obtained was 37% at iso-conversion of 6 ± 1% when BaGa-NaY was used at a temperature of 450 °C. The activity of the catalysts was directly proportional to the reducibility of the catalysts, which is in agreement with expectations.

Oxidative dehydrogenation of n-octane over Mg-containing SBA-15 material

In this study, it is proposed the incorporation of MgO into of SBA-15 via the direct synthesis different atomic ratios of Si/Mg (20, 40 and 80). Although the incorporation of Mg did not affect the mesoporous character of SBA-based materials, it was found dramatic changes in the SBA-15 characteristic honeycomb structure even after the incorporation of small amount of Mg. The creation of more basic sites in SBA-15, because of the addition of Mg, improved ODH conversion of n-octane with respect to SBA-15, used as reference. Sample with a Si/Mg molar ratio of 20 showed the best catalytic performance in the oxidative dehydrogenation of n-octane, while the sample with a Si/Mg molar ratio of 40 achieve the highest productions of olefins.

Effect of different weight loadings of MoO x /SBA-15 on the oxidative dehydrogenation of n-octane

This study examines the effect of different weight loadings of molybdenum as molybdenum oxide, supported on SBA-15 for the oxidative dehydrogenation of n-octane. The loadings of molybdenum on SBA-15 were performed by wet impregnation method the resultant catalysts were characterised by ICP-OES, BET physisorption studies, PXRD, Raman spectroscopy, TEM and SEM. Raman spectroscopy showed the presence of monomeric MoOx species at 4 wt%, monomeric and polymeric species at 7 wt%, polymeric and crystalline MoO3 species at 10 wt% and only crystalline Mo oxide at 18 wt%. SBA-15 as the catalyst is active but mainly selective to cracked products (33 %) and COx (32 %), whereas the loaded catalysts produced octenes and C8 aromatics. At similar conversion, the highest selectivity to octenes was observed with the 4 wt% catalyst which decreased with an increase in the molybdenum loadings. The highest selectivity to aromatics was observed with the 7 wt% catalyst. When the 10 wt% catalyst was tested over the temperature range of 350–550 °C, the conversion increased and selectivity towards aromatics was promoted. The extent of non-selective oxidation increased with an increase in the oxygen content in the feed while a variable GHSV study revealed that the octenes, cracked products and CO2 are the primary products of the reaction.

Octenes and Aromatics from the Oxidative Dehydrogenation of n-Octane over Co/TiO2 Catalysts

Titania supported cobalt catalysts with cobalt loadings between 5 and 25 wt% were synthesised via the wet impregnation technique and characterized by XRD, XPS, BET, SEM-EDX, TEM, ICP, TGA, Laser Raman and TPR analysis. Catalytic testing was carried out at a GHSV of 4,000 h−1 in a fixed bed reactor at atmospheric pressure, with air as the oxidant and nitrogen as the diluent from 350 to 550 °C. An n-octane to oxygen molar ratio of 1:1 was maintained. The dominant products were found to be the octenes, C8 aromatics and carbon oxides, with selectivities of up to 80 % to the former two products. Comparative studies based on the different metal loadings indicate that higher cobalt content enhanced the selectivities to octenes. COx formation was found to decrease significantly with an increase in temperature, as the pathway to octenes and C8 aromatics was favoured. TiO2 based cobalt catalysts transform n-octane, a model FT product, to octenes and C8 aromatics via oxidative dehydrogenation in a continuous flow fixed bed reactor with air as the oxidant and nitrogen the diluent, spanning the temperature range of 350–550 °C at atmospheric pressure.

The effect of the oxidation environment on the activity and selectivity to aromatics and octenes over cobalt molybdate in the oxidative dehydrogenation of n-octane

Varying the oxidative environment can cause phase changes in the catalyst which influence the product selectivity of octane activation.

Oxidative dehydrogenation of n-octane using vanadium pentoxide-supported hydroxyapatite catalysts

Vanadium pentoxide, with loadings varying from 2.5 to 15wt%, was supported on hydroxyapatite (HAp) by the wet impregnation technique. The materials were characterised by techniques such as X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP-OES), BET, FTIR, SEM, transmission electron microscopy (TEM), temperature programmed reduction (TPR) and temperature programmed desorption (TPD). From XRD and IR analyses, vanadium is found in the vanadium pentoxide phase for the lower loadings, whereas for weight loadings in excess of 10%, an additional pyrovanadate phase exists. Electron microscopy provides evidence of a homogenous distribution of the vanadium species on the hydroxyapatite. Oxidative dehydrogenation reactions carried out in a continuous flow fixed bed reactor showed that selectivity towards desired products was dependent on the vanadium concentration and the phase composition of the catalyst. Good selectivity towards octenes was achieved using the 2.5wt% V2O5 on HAp loaded catalyst. There was a marked decrease in octene selectivity and a significant increase in the formation of C8 aromatics when higher loadings of vanadium were used. At a conversion of 24% at 450°C, the 15wt% V2O5 on HAp showed a selectivity of 72% towards octenes. A maximum selectivity of 10% for C8 aromatics was obtained using the 15wt% V2O5 on HAp catalyst at a conversion of 36% at 550°C.

Oxidative dehydrogenation of n-octane using vanadium-magnesium oxide catalysts with different vanadium loadings

In this study, vanadium-magnesium oxide (VMgO) catalysts with different vanadium loadings were synthesized by the impregnation method and characterized by BET, ICP-OES, IR, powder XRD, TEM, SEM, TPR-H 2, and TPD of ammonia. The catalysts were tested for the oxidative dehydrogenation (ODH) of n-octane in a continuous flow fixed bed reactor at GHSV of 6000 h −1 and a temperature range of 350–550 °C using air as an oxidant and nitrogen as a make-up gas to give 9% n-octane in the gaseous mixture. The results showed that the vanadium concentration affects both textural and chemical properties of the catalysts and consequently their catalytic activity and selectivity towards the desired products (octenes and aromatics). The catalyst with a V 2O 5 concentration of 15% showed the best catalytic activity and productivity for the ODH products; especially for 1-octene and styrene.

The oxidative dehydrogenation of n-octane to styrene using catalysts derived from hydrotalcite-like precursors

Abstract A number of hydrotalcite-like compounds containing Mg and V were prepared with different Mg/V ratios, ranging from 1:1 to 4:1. The effect of variations in synthesis procedures, such as preparing the compounds under inert gas, hydrothermal treatments at different temperatures and different precipitating agents were examined. These compounds were characterised by ICP-OES, powder XRD (including, in part, in situ powder XRD under H 2 , air and N 2 ), Raman (incl. in situ ), IR, XPS and SEM. They were then investigated as catalysts for the aerobic oxidation of n -octane under varying conditions of GHSV, temperature and feed/air ratio. Styrene was found to be the dominant organic product and a maximum selectivity of 19%, corresponding to a yield of 14%, was obtained with a catalyst with a Mg/V ratio of 2.3. The mechanism appears to involve olefin formation, followed by cyclisation to form ethyl benzene, which is further dehydrogenated to form styrene. Catalysts where the precursors have more amorphous structures were found to be superior to those obtained from highly crystalline precursors, and catalysts derived from sodium salts were superior to those obtained from potassium salts.