ZnO Nanoparticles Encapsulated in Nitrogen-Doped Carbon Material and Silicalite-1 Composites for Efficient Propane Dehydrogenation

Chemistry; Catalysis; Nanoparticles © 2019 The Author(s)Non-oxidative propane dehydrogenation (PDH)is an attractive reaction from both an industrial and a scientific viewpoint because it allows direct large-scale production of propene and fundamental analysis of C-H activation respectively. The main challenges are related to achieving high activity, selectivity, and on-stream stability of environment-friendly and cost-efficient catalysts without non-noble metals. Here, we describe an approach for the preparation of supported ultrasmall ZnO nanoparticles (2–4 nm, ZnO NPs)for high-temperature applications. The approach consists of encapsulation of NPs into a nitrogen-doped carbon (NC)layer in situ grown from zeolitic imidazolate framework-8 on a Silicalite-1 support. The NC layer was established to control the size of ZnO NPs and to hinder their loss to a large extent at high temperatures. The designed catalysts exhibited high activity, selectivity, and on-stream stability in PDH. Propene selectivity of about 90% at 44.4% propane conversion was achieved at 600°C after nearly 6 h on stream. © 2019 The Author(s

Long-term changes in evapotranspiration over China and attribution to climatic drivers during 1980–2010

Evapotranspiration (ET) is one of the most important variables in terrestrial ecosystems, linking the carbon-water-energy cycles. In this study, we first analyze the spatial patterns of annual ET changes during 1980–2010 across China using four ET products: (i) the Global Land Evaporation Amsterdam Model version 3.0a (GLEAMv3.0), (ii) the EartH2Observe ensemble (EartH2Observe-En), (iii) the Global Land Data Assimilation System version 2.0 with Noah model (GLDAS2.0-Noah), and (iv) the Modern Era Retrospective-Analysis for Research and Application-Land (MERRA-Land). The results show that the spatial distribution of annual mean ET values and long-term changes derived from these four ET products are similar. Overall, large-scale increases in ET are observed in southeastern China, while decreases in ET over the northeast. Furthermore, we apply a newly developed separation method with the Budyko framework to quantify the individual contribution of five climatic factors to ET changes, including precipitation (P), net radiation (Rn), air temperature (T), vapour pressure deficit (VPD), and wind speed (u). It is found that the dynamics of P, Rn, and VPD are all strongly correlated with ET, suggesting that they are the major climatic factors influencing ET changes. Specifically, precipitation is the dominant factor for ET in water-limited regions, while ET changes in energy-limited regions are dominated by VPD according to all ET products except the EartH2Observe-En in which Rn and VPD have comparable performance. Our study highlights the importance of VPD in ET changes across energy-limited regions of China and suggests that the role of VPD in land surface-atmosphere interactions should be considered in future studies

Effect of supports on the kind of in-situ formed ZnOx_x species and its consequence for non-oxidative propane dehydrogenation

Non-oxidative propane dehydrogenation (PDH) to propene is the basis of various large-scale processes suffering however from high costs and environmental incompatibility of currently applied Pt- or Cr-containing catalysts. Herein, we demonstrate that active and selective catalysts can be obtained from cheap and commercially available Zr- or Ti-based supports and ZnO without producing any liquid or solid waste. Catalytically active species formed in situ under PDH conditions are composed of isolated ZnO$_x$ as concluded from X-ray absorption spectroscopic analysis. The kind of support affects the geometry of such species that is probably decisive for catalyst activity. ZnOx on the surface of LaZrO$_x$ revealed the highest Zn-related TOF of propene formation. However, the following activity order in terms of space time yield of propene formation (STY$_{C3H}$) at 550°C and about 50% equilibrium propane conversion using a feed with 40 vol% propane was obtained: ZnO//TiZrO$_x$ > ZnO//SiZrO$_x$ > ZnO//LaZrO$_x$ > ZnO//TiO$_2$. The best-performing catalyst showed STYC$_3$H$_6$ of 2 kg kg$_{cat}^-1$ h$^-1$ and was durable in 8 PDH/regeneration cycles. Temporal analysis of products with submillisecond resolution suggests that H$_2$ formation should be the rate-determining step in the course of the PDH reaction

The Fabrication of Ga2O3/ZSM-5 Hollow Fibers for Efficient Catalytic Conversion of n-Butane into Light Olefins and Aromatics

In this study, the dehydrogenation component of Ga2O3 was introduced into ZSM-5 nanocrystals to prepare Ga2O3/ZSM-5 hollow fiber-based bifunctional catalysts. The physicochemical features of as-prepared catalysts were characterized by means of XRD, BET, SEM, STEM, NH3-TPD, etc., and their performances for the catalytic conversion of n-butane to produce light olefins and aromatics were investigated. The results indicated that a very small amount of gallium can cause a marked enhancement in the catalytic activity of ZSM-5 because of the synergistic effect of the dehydrogenation and aromatization properties of Ga2O3 and the cracking function of ZSM-5. Compared with Ga2O3/ZSM-5 nanoparticles, the unique hierarchical macro-meso-microporosity of the as-prepared hollow fibers can effectively enlarge the bifunctionality by enhancing the accessibility of active sites and the diffusion. Consequently, Ga2O3/ZSM-5 hollow fibers show excellent catalytic conversion of n-butane, with the highest yield of light olefins plus aromatics at 600 °C by 87.6%, which is 56.3%, 24.6%, and 13.3% higher than that of ZSM-5, ZSM-5 zeolite fibers, and Ga2O3/ZSM-5, respectively

Controlling Reaction-Induced Loss of Active Sites in ZnOx_x/Silicalite-1 for Durable Nonoxidative Propane Dehydrogenation

ZnO-based catalysts are promising for nonoxidative propane dehydrogenation (PDH) to propene owing to their low cost and environmental friendliness but experience serious loss of the active component because of the reduction of ZnO to metallic Zn that evaporates. Here, we demonstrate that MgO-modified ZnO$_x$/silicalite-1 materials prepared through one-pot hydrothermal method are active, selective, and durable in the PDH reaction. The undesired loss of Zn could also be successfully suppressed without negative effect on the PDH performance owing to a strong interaction between Mg$^{2+}$ and ZnO$_x$, as concluded from the results of X-ray photoelectron and Fourier-transform infrared spectroscopic measurements as well as temperature-programmed reduction with CO. X-ray absorption spectroscopy revealed that atomically dispersed Zn$^{2+}$ sites are responsible for PDH. Using an industrially relevant feed with 40 vol % propane, propene selectivity between 88 and 95% at propane conversion between 15 and 32% was achieved over six PDH/oxidative regeneration cycles lasting for about 20 h on stream at 550 °C without loss in the initial activity, while some deactivation occurred after longer (up to about 60 h) time on stream. The deactivation (caused by Zn loss) constant of Mg-modified ZnO$_x$/silicalite-1 considering the 2nd and 20th cycles is more than 3 times lower than that of its Mg-free counterpart

TiO2_2-supported catalysts with ZnO and ZrO2_2 for non-oxidative dehydrogenation of propane: mechanistic analysis and application potential

Non-oxidative dehydrogenation of propane is one of the most promising technologies for propene production in terms of environmental impact and sustainability. The purpose of the present study was to develop environmentally friendly and low-cost catalysts alternatives to current applied Pt- or CrOx-based catalysts. Rutile TiO2-based catalysts with supported ZnOx and ZrOx species were established to show promising performance under industrially relevant conditions. The amount of propene produced within 3 h on propane stream at 550°C over the optimized catalyst with 2 wt% Zn and 5.6 wt% Zr is close to that obtained over commercial-like K-CrOx/Al2O3 and the state-of-the-art Cu/YZrOx catalyst. The selectivity to propene over our catalyst was about 95% at a propane conversion of about 23%. The kind of active sites and the effect of ZrO2 addition on catalyst performance and physicochemical properties were elucidated owing to the application of complementary characterisation techniques such as XRD, N2 physisorption, HRTEM, EDX, XPS, X-ray absorption spectroscopy, NH3-TPD, CO-TPR, Raman and O2-TPO. ZnOx clusters with 1-3 Zn atoms were concluded to be the active sites. ZrO2 enhances their intrinsic activity and inhibits the formation of inactive rhombohedral ZnTiO3 phase