Decoding Complexity in Structure-Sensitive Reactions

Structure-sensitive reactions involving the Mars and van Krevelen mechanism over metal and metal oxide catalysts are ubiquitous in reaction kinetics and engineering. The kinetic equations of such reactions are re-written to account for modern operando spectroscopy and microscopy observations. Emphasis is placed on reactions with nucleophilic (lattice) oxygen, oxygen reduction reversibility, an interconversion scheme, non-linear water adsorption, remote-control model, and non-uniform sites. The multiplicity of propane conversion over MoVTeNbOx catalysts is proven through a combination of non-linear competitive water adsorption, the presence of multiple active sites, a re-structuring active site, and oxygen adsorption. The modified remote-control kinetics for the Mars and van Krevelen mechanism can account for the observations of steady-state multiplicities and hysteresis. The results have implications for improving catalytic activity, reducing operating process costs, and active site engineering of selective oxidation catalysis.

In situ formation of ZnOx species for efficient propane dehydrogenation

Propane dehydrogenation (PDH) to propene is an important alternative to oil-based cracking processes, to produce this industrially important platform chemical1,2. The commercial PDH technologies utilizing Cr-containing (refs. 3,4) or Pt-containing (refs. 5–8) catalysts suffer from the toxicity of Cr(vi) compounds or the need to use ecologically harmful chlorine for catalyst regeneration9. Here, we introduce a method for preparation of environmentally compatible supported catalysts based on commercial ZnO. This metal oxide and a support (zeolite or common metal oxide) are used as a physical mixture or in the form of two layers with ZnO as the upstream layer. Supported ZnOx species are in situ formed through a reaction of support OH groups with Zn atoms generated from ZnO upon reductive treatment above 550 °C. Using different complementary characterization methods, we identify the decisive role of defective OH groups for the formation of active ZnOx species. For benchmarking purposes, the developed ZnO–silicalite-1 and an analogue of commercial K–CrOx/Al2O3 were tested in the same setup under industrially relevant conditions at close propane conversion over about 400 h on propane stream. The developed catalyst reveals about three times higher propene productivity at similar propene selectivity.

Oxidative Dehydrogenation of Propane over Vanadium Catalyst Supported on Alkali-Modifiedx-Al2O3

The propane oxidative dehydrogenation (ODH) reaction has been considered as an alternative method for propene production owing to its exothermic nature, which renders it environmentally friendly. The use of alkaline promoters for supported V catalysts can increase propene selectivity and partially inhibit the formation of CO and CO2. Our goal was to evaluate the promoting effect of K and Na and the support effect using gibbsite as precursor for the propane ODH reaction. Catalysts were prepared via co-impregnation of V and alkali metals on a previously prepared alumina support and were characterized using N2 adsorption-desorption, X-ray diffraction, temperature-programmed reduction, and isopropanol decomposition tests to evaluate their acid-base properties. The activity of the synthesized catalysts for the propane ODH reaction was evaluated at the O2:C3H8:He molar ratios of 5:2:4, 6:1:4, and 4:3:4. The addition of alkali metals to the V catalysts increased propane conversion and propene selectivity; moreover, both parameters increased with increasing molar fraction of O2 in the reactants. K doping increased the propene selectivity of the doped catalysts, because it inhibited a large fraction of catalytic surface acidic sites. A high molar fraction of O2 in the reactants facilitated the regeneration of the catalyst, whereas a high reoxidation rate improved catalytic activity and propene selectivity.

Relationship between Acidity and Activity on Propane Conversion over Metal-Modified HZSM-5 Catalysts

A systematic study of the comparative performances of different metal-impregnated HZSM-5 catalysts (Zn, Ga, Mo, Co, and Zr) for propane conversion is presented. The physicochemical properties of catalysts were characterized by means of XRD, BET, SEM, TEM, FTIR, XPS, 27Al MAS NMR, NH3-TPD and Py-FTIR. It was found that the acidities of the catalysts were significantly influenced by loading metal. More specifically, Mo-, Co- or Zr-modified catalysts showed a large metal size and low acidic density, resulting high olefin selectivity, while Zn- or Ga-modified catalysts maintained their small metal size and acidic density, and mainly reduced B/L due to the Lewis acid sites created by Zn or Ga species, resulting in high aromatics selectivity. Experimental results also showed that there is a balance between metals size and medium and strong acidity on propane conversion. Moreover, based on the different acidity of metal-modified HZSM-5 catalysts, the mechanism of propane conversion was also discussed.

Physicochemical Properties of MoVTeNb Mixed Oxide Catalysts Synthesized using Different Vanadium Sources

The use of different vanadium sources in the synthesis of multi-metal MoVTeNb oxide catalysts has been investigated for their effect on the physicochemical properties of catalysts. Metal oxides were synthesized by slurry method assisted with a microwave irradiation. Vanadium pentoxide (V2O5), vanadyl sulphate (VOSO4) and ammonium metavanadate (NH4VO3) were used as the vanadium sources, respectively. X-ray diffraction (XRD) pattern showed the existence of orthorhombic (M1) phases in all catalysts. The catalyst prepared using V2O5 produced the highest formation of the phase. This was further supported by Inductive Couple Plasma-Atomic Emission Spectroscopy (ICP-AES), which showed that the V2O5 catalyst has the highest V: Mo ratio, mainly responsible for the high catalytic activity. Temperature Programmed Reduction in Hydrogen (H2-TPR) showed better reducibility for the catalyst when compared to the others. Temperature Programmed Reaction (TPRn) confirmed that the oxidants active for propane conversion into acrylic acid were originated from the lattice of the catalyst.

Research into Thermocatalytic Propane-to-Propylene Synthesis Using Iron-Containing Composite Carbon Materials

The development of modern thermocatalytic technologies for processing oil and gas raw materials is one of the promising areas for the advancement of chemical production. New highly efficient catalytic systems with the required technical characteristics and long service life play an essential role in solving these issues. The paper focuses on obtaining propylene by selective propane dehydrogenation. In the course of the experiments, composite iron-containing catalysts were synthesized, in which the active component is iron oxide in combination with an inert carbon matrix. FAS activated carbon and carbon nanotubes were used as the matrix. As a result of the synthesis on the catalyst surface it was possible to obtain catalytic centers that transfer electrons by changing the degree of iron oxidation when converting the starting materials into the target reaction products. Findings of research show that the obtained iron-containing catalysts significantly increase the efficiency of the process in comparison with the efficiency of thermal cracking of propane. Thus, the Fe3+/FAS catalyst showed a conversion rate of the initial reagent of 68 % and a propylene selectivity of about 42 %. Further transition to catalytic systems based on singlelayer and double-layer carbon nanotubes modified with iron oxide (Fe3+/CNTI and Fe3+/CNTII) made it possible to obtain propane conversion up to 37--40 % with a decrease in propylene selectivity to 29--30 %. Studies of the service life of the synthesized catalytic systems and the possibility of their regeneration show that, with account for the regeneration, the activity of the catalysts and the main technical characteristics of the propane-to-propylene cracking process remain unchanged for 10 working cycles

Propane Steam Reforming over Catalysts Derived from Noble Metal (Ru, Rh)-Substituted LaNiO3 and La0.8Sr0.2NiO3 Perovskite Precursors

The propane steam reforming (PSR) reaction was investigated over catalysts derived from LaNiO3 (LN), La0.8Sr0.2NiO3 (LSN), and noble metal-substituted LNMx and LSNMx (M = Ru, Rh; x = 0.01, 0.1) perovskites. The incorporation of foreign cations in the A and/or B sites of the perovskite structure resulted in an increase in the specific surface area, a shift of XRD lines toward lower diffraction angles, and a decrease of the mean primary crystallite size of the parent material. Exposure of the as-prepared samples to reaction conditions resulted in the in situ development of new phases including metallic Ni and La2O2CO3, which participate actively in the PSR reaction. The LN-derived catalyst exhibited higher activity compared to LSN, and its performance for the title reaction did not change appreciably following partial substitution of Ru for Ni. In contrast, incorporation of Ru and, especially, Rh in the LSN perovskite matrix resulted in the development of catalysts with significantly enhanced catalytic performance, which improved by increasing the noble metal content. The best results were obtained for the LSNRh0.1-derived sample, which exhibited excellent long-term stability for 40 hours on stream as well as high propane conversion (XC3H8 = 92%) and H2 selectivity (SH2 = 97%) at 600 °C.

Effect of Operating Conditions on the Performance of Rh/TiO2 Catalyst for the Reaction of LPG Steam Reforming

The catalytic performance of Rh/TiO2 catalyst was investigated for the reaction of Liquefied Petroleum Gas (LPG) steam reforming with respect to the operating conditions employed. The impacts of reaction temperature, steam/C ratio, Gas Hourly Space Velocity (GHSV), and time were examined and discussed both in the absence and presence of butane in the feed. It was found that the catalytic performance is improved by increasing the reaction temperature, steam content in the feed, and/or by decreasing GHSV. In the presence of butane in the feed, the effect of H2O/C ratio on catalytic performance is prominent, whereas the opposite was observed for the effect of GHSV. The propane conversion curve decreases by adding butane in the feed, indicating that the presence of butane retards propane steam reforming. The investigation of the dynamic response of Rh/TiO2 catalyst to variations of H2O/C ratio showed that neither catalytic activity nor product selectivity is varied with time following abrupt changes of the steam/C ratio between 2 and 7. The catalyst exhibited excellent stability with time-on-stream at 500 and 650 °C. However, a reversible catalyst deactivation seems to be operable when the reaction occurs at 600 °C, resulting in a progressive decrease of propane conversion, which, however, can be completely restored by increasing the temperature to 650 °C in He flow, respectively. The long-term stability of Rh/TiO2 catalyst in the form of pellets showed that this catalyst is not only active and selective but also stable, and therefore, it is a promising catalyst for the reaction of LPG steam reforming.

Kinetic Features of the Process of Conversion of Propane to Aromatic Hydrocarbons

Kinetic features of the propane conversion into aromatic hydrocarbons over a gallium-containing zeolite catalyst have been investigated. On the basis of the experimentally obtained kinetic dependences and the available literature data, a kinetic model of propane aromatization is proposed, which makes it possible to form various variations of chemical reaction behavior and to calculate the most probable routes of propane conversion

Development of V-Based Oxygen Carriers for Chemical Looping Oxidative Dehydrogenation of Propane

Two different preparation methods, viz. incipient impregnation and mechanical mixing, have been used to prepare V-based oxygen carriers with different V loadings for chemical looping oxidative dehydrogenation of propane. The effect of the preparation method, V loading, and reaction temperature on the performance of these oxygen carriers have been measured and discussed. It was found that the VOx species can be well distributed on the support when the V loading is low (5 wt.% and 10 wt.%), but they may become aggregated at higher loadings. For oxygen carriers with a higher V loading, the oxygen transport capacity of the oxygen carrier, propane conversion and COx selectivities increase, while the propylene selectivity decreases. With a V-loading of 10 wt.%, the maximum propylene yield was achieved. The VOx species were better distributed over the support when applying the impregnation method; however, at higher V loadings the V-based oxygen carriers prepared by mechanical mixing showed a larger oxygen transport capacity. The oxygen carriers prepared by impregnation showed a better performance for the oxidative dehydrogenation of propane (ODHP) and re-oxidation reactions compared to oxygen carriers prepared by mechanical mixing. Higher reaction temperatures are favorable for the re-oxidation reaction, but unfavorable for the propylene production.