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Surface or bulk promotion of ZrO2-based catalysts with Bi2O3 facilitates the removal of lattice oxygen from ZrO2 under reductive conditions resulting in the formation of coordinatively unsaturated Zr cations. The catalysts demonstrated an industrially relevant propene yield at 600 °C. The results highlight the importance of the usage of suitable promoters for controlling catalyst performance. [ABSTRACT FROM AUTHOR]

Conversion of propane or butanes from natural/shale gas into propene or butenes, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes. Herein, we critically analyse recent developments in the non-oxidative, oxidative, and CO2-mediated dehydrogenation of propane and isobutane to the corresponding olefins over metal oxide catalysts. Particular attention is paid to (i) comparing the developed catalysts in terms of their application potential, (ii) structure–activity–selectivity relationships for tailored catalyst design, and (iii) reaction-engineering aspects for improving product selectivity and overall process efficiency. On this basis, possible directions for further research aimed at the development of inexpensive and environmentally friendly catalysts with industrially relevant performance were identified. In addition, we provide general information regarding catalyst preparation and characterization as well as some recommendations for carrying out non-oxidative and CO2-mediated dehydrogenation reactions to ensure unambiguous comparison of catalysts developed in different studies. [ABSTRACT FROM AUTHOR]

Bulk CrZrO x , supported CrO x /LaZrO x , and CrO x /Al 2 O 3 catalysts were prepared with Cr surface site density varying between 0.04 and 2.3 nm −2 . They were characterized by BET, XRD, temperature-programmed reduction with H 2 , and NH 3 temperature-programmed desorption. Their activity, selectivity, and durability in non-oxidative propane dehydrogenation (PDH) were determined under industrially relevant conditions. The catalysts on the basis of ZrO 2 outperform their Al 2 O 3 -based counterparts in terms of PDH activity. This was explained by the presence of two catalytically active sites, i.e. CrO x and coordinatively unsaturated Zr 4+ (Zr cus ) sites. The former species promote formation of Zr cus under reaction conditions or upon reductive catalyst treatment. Practical relevance of CrZrO x and CrO x /LaZrO x catalysts was checked in a series of 10 PDH/regeneration cycles performed at 550 °C. The catalysts possessing up to 40 times lower chromium content performed superior to an analogue of commercial K-CrO x /Al 2 O 3 with respect to space time yield of propene and the amount of propene formed within one PDH cycle. In terms of durability, bulk promoting of ZrO 2 with Cr was found to be advantageous over simple deposition of CrO x on the surface of LaZrO x . [ABSTRACT FROM AUTHOR]

We elucidated the effects of H 2 and the kind (Cu or Ru) of supported metal on product selectivity and on-stream stability of MZrO x -based catalysts (M = La or Y) in the non-oxidative dehydrogenation of propane to propene. Coke formation was hindered in hydrogen presence. However, the selectivity to C 1 -C 2 hydrocarbons increased over Ru/MZrO x but did not change over Cu/LaZrO x. This difference was related to the low activity of Cu for cracking and hydrogenolysis reactions. Using feed with 40 vol% C 3 H 8 and 30 vol% H 2 in N 2 , Cu/LaZrO x showed stable conversion (20%) and propene selectivity (88%) within 4.5 h on stream at 550 °C. Unlabelled Image • C 3 H 8 dehydrogenation was studied over (Cu/Ru)/MZrO x (M = La/Y) in H 2 presence. • H 2 improves catalyst on-stream stability due to suppressing coke formation. • Selectivity to cracking products is affected by H 2 content and supported metal. • Cu reveals lower activity for cracking/hydrogenolysis than Ru. [ABSTRACT FROM AUTHOR]

The effect of Cs-, Ca- and P-containing promoters for binary Cr-Zr-O x on propene selectivity and catalyst stability with time-on-stream in the non-oxidative propane dehydrogenation was elucidated. Large improvements in this catalytic performance were achieved when combining Cs and P promoters. They were suggested to partially block dehydrogenation sites resulting in the isolation of the latter. As a reason thereof, coke formation through propene oligomerization is inhibited in favor of propene desorption from the catalyst surface. The present catalysts achieved ~30% conversion of propane at WHSV (C 3 H 8) of 5.89 h-1 at 550 °C in comparison with 20% over K-CrO x /Al 2 O 3. The selectivity to propene was determined to be ~94%. Unlabelled Image • Promoted CrZrOx materials were tested for non-oxidative propane dehydrogenation. • Catalytic performance was improved significantly by promoting with Cs, Ca and/or P. • Coke formation is the main reason for catalyst deactivation. • Isolating active sites by Cs and/or P inhibits coke formation. [ABSTRACT FROM AUTHOR]

• CrZrO x /SiO 2 with different degree of ZrO 2 crystallinity were prepared and tested in non-oxidative propane dehydrogenation. • Their activity increases with the crystallinity, while the selectivity to propene is not affected. • Coke formation is the main reason for catalyst deactivation. • CrZrO x /SiO 2 showed good durability in 10 PDH/regeneration cycles under industrial relevant conditions. A series of SiO 2 -supported CrZrO x -based catalysts with same Cr content but different degrees of ZrO 2 crystallinity were synthesized by a simple impregnation-calcination method. The catalysts were studied for their activity, selectivity and on-stream stability in non-oxidative propane dehydrogenation (PDH) under industrially relevant conditions. They were characterized by complementary techniques such as XRD, BET, NH 3 -TPD, H 2 -TPR, operando UV–vis, EPR, Raman spectroscopy and TPO. The results demonstrate that ZrO 2 crystallinity plays an important role in propene formation. Although catalyst activity can be significantly augmented upon increasing the degree of crystallinity of ZrO 2 , no decrease in the selectivity could be observed at the same degree of propane conversion, i.e. 92 % at about 20 % conversion. The catalysts showed good durability in 10 PDH/oxidative regeneration cycles at 550 °C. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co/ZrO 2 catalysts with single Co2+ are efficient for C 3 H 8 dehydrogenation to C 3 H 6. • 1Co/ZrO 2 performed durable over multiple dehydrogenation/regeneration cycles. • 1Co/ZrO 2 showed higher activity than a commercial analogue of K-CrO x /Al 2 O 3. • The high activity results from a synergy effect between Co2+ and Zr cus. • Recombination of surface H species is the rate-limiting step in C 3 H 6 formation. Due to the increased production of shale gas containing propane, the share of non-oxidative propane dehydrogenation (PDH) in the large-scale production of propene is expected to continue to grow. There are, however, some ecofriendly and cost shortcomings associated with the currently applied Cr- or Pt-containing catalysts. Against this background, we present here Co/ZrO 2 alternatives and reveal the fundamentals affecting their activity, which can be used for purposeful catalyst design. Co2+ species homogeneously distributed within the lattice of ZrO 2 significantly increase the activity of coordinatively unsaturated Zr4+ for the PDH reaction. The increase is caused by accelerating both the cleavage of CH bonds in propane and the recombination of surface H species, with the latter reaction being the rate-limiting step. The best-performing catalyst outperformed an analogue of commercial K-CrO x /Al 2 O 3 in terms of the rate of propene formation and demonstrated durable performance over a series of 10 PDH/regeneration cycles under industrially relevant conditions. It also outperformed most previously developed Co-containing catalysts in terms of propene productivity. The space–time yield of propene formation achieved at 57 % equilibrium propane conversion at 550 °C was 0.71 kg·h−1·kg cat -1. [ABSTRACT FROM AUTHOR]

• Differently prepared bare ZrO 2 materials were tested in non-oxidative C 3 H 8 dehydrogenation. • Their activity and selectivity depends on the ability of ZrO 2 to release lattice oxygen under reducing conditions. • This catalyst property can be tuned by preparation method. • Controlled decomposition of ZrO(NO 3) 2 results in ZrO 2 with improved reducibility and high activity and selectivity. A detailed study was carried out on the factors affecting the activity and, particularly, selectivity of bare ZrO 2 in the non-oxidative propane dehydrogenation (PDH) to propene. To this end, a series of ZrO 2 samples differing in their redox and acidic properties, morphology and phase composition were synthesized by precipitation, sol-gel, glycine-nitrate combustion, template, digestion and calcination methods as well as through controlled thermal decomposition of zirconium oxonitrate. The ability of ZrO 2 to release its lattice oxygen in reducing atmosphere was identified as the main factor affecting PDH activity and propene selectivity. This is due to the fact that so generated surface zirconium cations (Zr cus) with lower coordination than their regular counterparts show higher activity for propene formation. As the latter sites strongly contribute to coke formation, the selectivity to this undesired product can be hindered through improving the ability of ZrO 2 to generate Zr cus sites under reducing conditions. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Monoclinic or tetragonal ZrO 2 were tested for non-oxidative C 3 H 8 dehydrogenation. • Propene selectivity and formation rate increase with a decrease in crystallite size. • DFT calculations predict same active sites for monoclinic and tetragonal ZrO 2. • Monoclinic ZrO 2 performs superior to tetragonal ZrO 2. • The effect of the phase composition is related to the number of active sites. Abstract A series of bare ZrO 2 materials composed either of monoclinic or tetragonal phase with the size of crystallites varying between 3.7 and 43.4 nm were prepared by precipitation, hydrothermal or thermal calcination methods. The samples were characterized by XRD, BET, CO-TPR, NH 3 -TPD, electrical conductivity measurements and operando UV–vis spectroscopy. Density functional theory calculations provided molecular insights into the kind of active site and individual steps of propane dehydrogenation to propene and hydrogen. Regardless of the phase composition, two Zr cations located at an oxygen vacancy, i.e. coordinatively unsaturated Zr cations (Zr cus sites) having, nevertheless, different chemical environment in tetragonal and monoclinic ZrO 2 , were concluded to be responsible for homolytic breaking of C H bonds in propane. Monoclinic ZrO 2 showed, however, higher rate of propene formation and higher propene selectivity than ZrO 2 stabilized in the tetragonal phase. Both the activity and the selectivity to propene increased with a decrease in the size of crystallites. The effects of phase composition and crystallite size on the PDH performance were related to the ability of ZrO 2 to release lattice oxygen upon reductive catalyst treatment and thus to create Zr cus sites. The knowledge derived can be used for further optimizing PDH performance of ZrO 2 -based catalysts and also extended to other non-reducible metal oxides. [ABSTRACT FROM AUTHOR]

[Display omitted] • Metallic Co0 species in situ formed are suggested to be the main active sites for PDH. • CoO x but not Co0 is responsible for the formation of undesired products. • Transformation of CoO x to Co0 depends on the strength of CoO x -support interactions. • Co/Silicalite-1 showed the C 3 H 6 space time yield of 1.50 kg·h−1·kg cat −1 at 550 °C. Although Co-containing catalysts show attractive performance in the non-oxidative propane dehydrogenation to propylene (PDH), their tailored development is hindered by the ambiguity in the kind of active sites (Co0 or Co2+O x) responsible for selective and unselective pathways. Herein, we demonstrate that supported CoO x species expends its lattice oxygen through oxidation of C 3 H 8 to CO 2 /C 3 H 6 and are accordingly transformed into Co0 species. The latter are decisive for the selective dehydrogenation of propane to propylene with high activity. The ability of CoO x to in situ form Co0 is affected by metal-oxide-support interactions (MOSI). In addition to the kind of support, defective OH groups (OH nests) in ZSM-5-type zeolites seem to be relevant for the MOSI effect. In comparison with previous studies highlighting the importance of the size of CoO x species and their acidity for coke formation, we could not establish any direct correlation. Nevertheless, the presence of CoO x seems to be relevant for this undesired reaction because the formation of coke is hindered when CoO x species are transformed to Co0 species. Catalyst acidic sites are responsible for cracking reactions. Thus, the derived mechanistic insights reveal the nature of active sites both for selective and unselective pathways and provide fundamentals for catalyst development. From an industrial viewpoint, the developed Co/Silicalite-1 (Co/S-1) catalyst with the weakest MOSI and accordingly highest fraction of Co0 showed the space time yield of propylene formation of 1.50 kg·h−1·kg cat −1 at 67 % equilibrium propane conversion and propylene selectivity of 95.3 % and the Co-related TOF value of 0.145 s−1 at 550 °C. These both activity values exceed those of the state-of-the-art Co-containing catalysts. The productivity of Co/S-1 is comparable or even higher than that of industrially relevant Pt- or Cr-containing catalysts. [ABSTRACT FROM AUTHOR]