Elucidating the Nature of Active Sites and Fundamentals for their Creation in Zn-Containing ZrO2–Based Catalysts for Nonoxidative Propane Dehydrogenation

Well-Defined Supported ZnOx Species: Synthesis, Structure, and Catalytic Performance in Nonoxidative Dehydrogenation of C3-C4 Alkanes

ConspectusZinc oxide (ZnO) is a multipurpose material and finds its applications in various fields such as rubber manufacturing, medicine, food additives, electronics, etc. It has also been intensively studied in photocatalysis due to its wide band gap and environmental compatibility. Recently, heterogeneous catalysts with supported ZnOx species have attracted more and more attention for the dehydrogenation of propane (PDH) and isobutane (iBDH) present in shale/natural gas. The olefins formed in these reactions are key building blocks of the chemical industry. These reactions are also of academic importance for understanding the fundamentals of the selective activation of C-H bonds. Differently structured ZnOx species supported on zeolites, SiO2, and Al2O3 have been reported to be active for nonoxidative dehydrogenation reactions. However, the structure-activity-selectivity relationships for these catalysts remain elusive. The main difficulty stems from the preparation of catalysts containing only one kind of well-defined ZnOx species.In this Account, we describe the studies on PDH and iBDH over differently structured ZnOx species and highlight our approaches to develop catalysts with controllable ZnOx speciation relevant to their performance. Several methods, including (i) the in situ reaction of gas-phase metallic Zn atoms with OH groups on the surface of supports, (ii) one-pot hydrothermal synthesis, and (iii) impregnation/anchoring methods, have been developed/used for the tailored preparation of supported ZnOx species. The first method allows precise control of the molecular structure of ZnOx through the nature of the defective OH groups on the supports. Using this method, a series of ZnOx species ranging from isolated, binuclear to nanosized ZnOx have been successfully generated on different SiO2-based or ZrO2-based supports as demonstrated by complementary ex/in situ characterization techniques. Based on kinetic studies and detailed characterization results, the intrinsic activity (Zn-related turnover frequency) of ZnOx was found to depend on its speciation. It increases with an increasing number of Zn atoms in a ZnmOn cluster from 1 to a few atoms (less than 10) and then decreases strongly for ZnOx nanoparticles. The latter promote the formation of undesired C1-C2 hydrocarbons and coke, resulting in lower propene selectivity in comparison with the catalysts containing only ZnOx species ranging from isolated to subnanometer ZnmOn clusters. In addition, the strategy for improving the thermal stability of ZnOx species and the consequences of mass-transport limitations for DH reactions were also elucidated. The results obtained allowed us to establish the fundamentals for the targeted preparation of well-structured ZnOx species and the relationships between their structures and the DH performance. This knowledge may inspire further studies in the field of C-H bond activation and other reactions, in which ZnOx species act as catalytically active sites or promoters, such as the dehydroaromatization of light alkanes and the hydrogenation of CO2 to methanol.

Zinc Oxide Nanoclusters Encapsulated in MFI Zeolite as a Highly Stable Adsorbent for the Ultradeep Removal of Hydrogen Sulfide

Often, trace impurities in a feed stream will cause failures in industrial applications. The efficient removal of such a trace impurity from industrial steams, however, is a daunting challenge due to the extremely small driving force for mass transfer. The issue lies in an activity-stability dilemma, that is, an ultrafine adsorbent that offers a high exposure of active sites is favorable for capturing species of a low concentration, but free-standing adsorptive species are susceptible to rapidly aggregating in working conditions, thus losing their intrinsic high activity. Confining ultrafine adsorbents in a porous matrix is a feasible solution to address this activity-stability dilemma. We herein demonstrate a proof of concept by encapsulating ZnO nanoclusters into a pure-silica MFI zeolite (ZnO@silicalite-1) for the ultradeep removal of H2S, a critical need in the purification of hydrogen for fuel cells. The Zn species and their interaction with silicalite-1 were thoroughly investigated by a collection of characterization techniques such as HADDF-STEM, UV-visible spectroscopy, DRIFTS, and 1H MAS NMR. The results show that the zeolite offers rich silanol defects, which enable the guest nanoclusters to be highly dispersed and anchored in the silicious matrix. The nanoclusters are present in two forms, Zn(OH)+ and ZnO, depending on the varying degrees of interaction with the silanol defects. The ultrafine nanoclusters exhibit an excellent desulfurization performance in terms of the adsorption rate and utilization. Furthermore, the ZnO@silicalite-1 adsorbents are remarkably stable against sintering at high temperatures, thus maintaining a high activity in multiple adsorption-regeneration cycles. The results demonstrate that the encapsulation of active metal oxide species into zeolite is a promising strategy to develop fast responsive and highly stable adsorbents for the ultradeep removal of trace impurities.

Prediction of Stable Surfaces of Metal Oxides through the Unsaturated Coordination Index

This study proposes the unsaturated coordination index, σ, as a potential descriptor of the stability of metal-oxide surfaces cleaved from bulk. The value of σ, the number of missing bonds per unit area, can be obtained very quickly using only crystallographic data, namely, the bulk geometry. The surface energies of various binary oxides, with and without atom relaxation, were calculated. Their correlations with σ had good coefficients of determination (R2) values, particularly in high-symmetry crystals. The proposed descriptor is very useful for an initial evaluation of stable metal-oxide surfaces without conducting any surface model calculations.

Probing the Nature of Zinc in Copper-Zinc-Zirconium Catalysts by Operando Spectroscopies for CO2 Hydrogenation to Methanol

Active Zn species in Cu-based methanol synthesis catalysts have not been clearly identified yet due to their complex nature and dynamic structural changes during reactions. Herein, atomically dispersed Zn on ZrO₂ support is established in Cu-based catalysts by separating Zn and Zr components from Cu (Cu-ZnZr) via the double-nozzle flame spray pyrolysis (DFSP) method. It exhibits superiority in methanol selectivity and yield compared to those with Cu-ZnO interface and isolated ZnO nanoparticles. Operando X-ray absorption spectroscopy (XAS) reveals that the atomically dispersed Zn species are induced during the reaction due to the strengthened Zn-Zr interaction. They can suppress formate decomposition to CO and decrease the H₂ dissociation energy, shifting the reaction to methanol production. This work enlightens the rational design of unique Zn species by regulating coordination environments and offers a new perspective for exploring complex interactions in multi-component catalysts.

Catalyst supports based on ZnO-ZnAl2O4 nanocomposites with enhanced selectivity and coking resistance in isobutane dehydrogenation

Development of coking resistant supports and catalysts for hydrocarbons conversion is challenging, especially when using such acidic materials as alumina. Apparently, this problem can be mitigated by using spinels that are less acidic, being, however, thermally stable. In this study, a series of ZnO-ZnAl₂O₄ nanocomposites with different ZnO loading were prepared by urotropine-nitrate combustion synthesis to be used as supports for isobutane dehydrogenation catalysts. The nanocomposites were characterized by XRD, SEM, N₂-physisorption analysis, EDS, H₂-TPR, TPD of NH₃ and tested in isobutane dehydrogenation. Spinels with small amounts of ZnO displayed higher acidity and specific surface areas than samples with a higher ZnO content (30-40 mol%). At the same time, the maximum activity and the lowest selectivity to by-products (CH₄ and C₃H₆) after 10 min of the reaction were observed for the nanocomposite containing 20 mol% of ZnO. The obtained nanocomposites have demonstrated better resistance to coking compared to commercial alumina.

Advanced design and development of catalysts in propane dehydrogenation

Propane dehydrogenation (PDH) is an industrial technology for direct propylene production, which has received extensive attention and realized large-scale application. At present, the commercial Pt/Cr-based catalysts suffer from fast deactivation and inferior stability resulting from active species sintering and coke depositing. To overcome the above problems, several strategies such as the modification of the support and the introduction of additives have been proposed to strengthen the catalytic performance and prolong the robust stability of Pt/Cr-based catalysts. This review firstly gives a brief description of the development of PDH and PDH catalysts. Then, the advanced research progress of supported noble metals and non-noble metals together with metal-free materials for PDH is systematically summarized along with the material design and active origin as well as the existing problems in the development of PDH catalysts. Furthermore, the review also emphasizes advanced synthetic strategies based on novel design of PDH catalysts with improved dehydrogenation activity and stability. Finally, the future challenges and directions of PDH catalysts are provided for the development of their further industrial application.

Porous Single-Crystalline Monolith to Enhance Catalytic Activity and Stability

Engineering the catalytic activity and stability of materials would require the identification of the structural features that can tailor active sites at surfaces. Porous single crystals combine the ordered lattice structures and disordered interconnected pores, and they would therefore provide the advantages of precise structure features to identify and engineer the active sites at surfaces. Herein, we fabricate porous single-crystalline vanadium nitride (VN) at centimeter scale and further dope Fe (Fe_0.1V_0.9N) and Co (Co_0.1V_0.9N) in lattice to engineer the active sites at surface. We demonstrate that the active surface is composed of unsaturated coordination of V-N, Fe-N, and Co-N structures which lead to the generation of high-density active sites at the porous single-crystalline monolith surface. The interconnected pores aid the pore-enhanced fluxion to facilitate species diffusion in the porous architectures. In the nonoxidative dehydrogenation of ethane to ethylene, we demonstrate the outstanding performance with ethane conversion of 36% and ethylene selectivity of 99% at 660°C. Remarkably stability as a result of their single-crystalline structure, the monoliths achieve the outstanding performance without degradation being observed even after 200 hours of a continuous operation in a monolithic reactor. This work not only demonstrates the effective structural engineering to simultaneously enhance the stability and overall performance for practically useful catalytic materials but also provide a new route for the element doping of porous single crystals at large scale for the potential application in other fields.

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.

High-Density Lewis Acid Sites in Porous Single-Crystalline Monoliths to Enhance Propane Dehydrogenation at Reduced Temperatures

The non-oxidative dehydrogenation of propane to propylene plays an important role in the light-olefin chemical industry. However, the conversion and selectivity remain a fundamental challenge at low temperatures. Here we create and engineer high-density Lewis acid sites at well-defined surfaces in porous single-crystalline Mo₂ N and MoN monoliths to enhance the non-oxidative dehydrogenation of propane to propylene. The top-layer Mo ions with unsaturated Mo-N_1/6 and Mo-N_1/3 coordination structures provide high-density Lewis acid sites at the surface, leading to the effective activation of C-H bonds without the overcracking of C-C bonds during the non-oxidative dehydrogenation of propane. We demonstrate a propane conversion of ≈11 % and a propylene selectivity of ≈95 % with porous single-crystalline Mo₂ N and MoN monoliths at 500 °C.

Recent progress in heterogeneous metal and metal oxide catalysts for direct dehydrogenation of ethane and propane

Metal and metal oxide catalysts for non-oxidative ethane/propane dehydrogenation are outlined with respect to catalyst synthesis, structure–property relationship and catalytic mechanism.

Propane dehydrogenation over PtZn localized at Ti sites on TS-1 zeolite

PtZn localized at Ti sites on TS-1 zeolite exhibited high activity (58.09 mol_C3H6 g_Pt⁻¹ h⁻¹) and durability (k_d = 0.0158 h⁻¹, 96.8 h time on stream at 600 °C) for propane dehydrogenation.

Current status and perspectives in oxidative, non-oxidative and CO2-mediated dehydrogenation of propane and isobutane over metal oxide catalysts

Conversion of propane or isobutane from natural/shale gas into propene or isobutene, which are indispensable for the synthesis of commodity chemicals, is an important environmentally friendly alternative to oil-based cracking processes.

Untangling the active sites in the exposed crystal facet of zirconium oxide for selective hydrogenation of bioaldehydes

A quantitative yield of furfuryl alcohol from furfural in isopropanol was achieved with ZrO₂ having monoclinic phase with a maximum exposed (−111) facet, whereas ZrO₂ possessing predominantly tetragonal phase yielded only 8.0% of furfuryl alcohol.