22 Search Results

Catalytic hydrogenolysis and hydrocracking are promising technologies for the conversion of polyolefin waste into hydrocarbons. Here, to explore the economic and environmental viability of these approaches, we conducted techno-economic analysis and life cycle assessment of polyethylene hydrogenolysis and hydrocracking processes that produce fuel-range products or olefins, respectively. Relative to primary production, the minimum selling price (MSP) of alkanes for linear-chain naphtha was estimated to be 1.8-fold higher, whereas branched-chain naphtha and propylene were shown to be cost competitive. Environmental impacts showed similar trends across scenarios, with propylene production by hydrocracking exhibiting greenhouse gas emissions comparable with conventional propylene due tomore » relatively low hydrogen demand and high yield. Together, these results suggest that innovations in catalysis and reaction engineering will be required to make viable products that are scale matched with today's plastics, such as light olefins for closed-loop recycling.« less

The molecular structure and activity of supported MoO_x olefin metathesis catalysts are heavily impacted by the choice of catalyst support. In this study, surface modification of the SiO₂ support with AlO_x and selective anchoring of the MoO_x on the surface AlO_x sites were used to tune the structure, activation, and reactivity of the resulting surface MoO_x sites. Extensive in situ molecular characterization, chemical probe studies, and density functional theory (DFT) calculations reveal that the enhanced activity of the supported MoO_x/AlO_x/SiO₂ catalyst over the MoO_x/ SiO₂ catalyst is associated with more favorable activation and kinetics of surface MoO_x anchored at AlO_xmore » sites.« less

Propylene oxide, a key commodity of the chemical industry for a wide range of consumer products, is synthesized through sequential propane dehydrogenation and epoxidation reactions. However, the lack of a direct catalytic route from propane to propylene oxide reduces efficiency and represents a major challenge for catalysis science. Herein, we report the discovery of a highly active and selective catalyst, made of alumina-supported subnanometer copper clusters, which can directly convert propane to propylene oxide at temperatures as low as 150 °C. Moreover, at higher temperatures, on the same catalysts, the selectivity is switched to propylene. Accompanying theoretical calculations indicate thatmore » partially oxidized and/or hydroxylated clusters have low activation energies for both propane dehydrogenation and propylene epoxidation pathways, enabling direct conversion with very high selectivity for propylene oxide. The discovery of a low-temperature catalyst that can convert propane directly to propylene oxide provides an important opportunity for the development of energy-efficient and economic catalysts for this industrially critical process. Similarly, when operating at higher temperatures, these catalysts are posed as potent oxidative dehydrogenation catalysts.« less

Here, we show that the electronic environment around active Mo centers supported on mesoporous silicates can be tuned by the addition of transition metals creating highly dispersed bimetallic catalysts that display enhanced activity for ethylene + 2-butene metathesis to propylene. The bimetallic catalysts are prepared by incorporating electrophilic Lewis acid metals (M) such as Nb, Ta, Zr, or Hf as dopant promoters into mesoporous KIT-6 supports using a one-pot sol–gel technique followed by impregnation of the Mo species. All the bimetallic Mo/M-KIT-6 catalysts display better activity than monometallic Mo/KIT-6 catalyst (28.7 ± 1.1 mmol (mol_{Mo ^s})^–1), with (Mo/Nb-KIT-6) catalysts exhibitingmore » maximum propylene formation rates (54.2 ± 0.5 mmol (mol_{Mo ^s})^–1) at an identical Mo loading.« less

Nonoxidative dehydrogenation of propane to propylene using Pt-based supported catalysts is an active research area in catalysis because catalyst attributes of Pt sites can be controlled by careful design of active sites. One way to achieve this is by the addition of a second metal that may impart a change in the electron density of active sites, which in turn affects catalytic performance. In this study, bimetallic Pt and B sites were deposited on powder SiO2 using atomic layer deposition (ALD). Boron was first deposited on SiO2 via half-cycle ALD using triisoproplyborate as the B source. Following calcination, Pt depositionmore » was performed via half-cycle ALD using trimethyl(methylcyclopentadienyl)platinum(IV) as the Pt source. The synthesized catalysts were reduced under H2 at 550 °C and characterized using inductively coupled plasma optical emission spectroscopy for elemental analysis, diffuse reflectance infrared Fourier transform spectroscopy of adsorbed CO to examine the properties of Pt, and time-resolved X-ray absorption near edge structure spectroscopy to examine the changes in the reducibility of Pt sites. The samples were then tested for nonoxidative dehydrogenation of propane at 550 °C using a fixed-bed plug-flow reactor to examine the role of B on the catalytic performance. Characterization results showed that the addition of B imparted an increase in electron density and affected the reducibility of Pt sites. In addition, incorporating B on SiO2 created anchoring sites for Pt ALD. The amount of Pt deposited on B/SiO2 was 2.2 times that on SiO2. Catalytic activity results revealed the addition of B did not change the initial activity of Pt sites significantly, but improved propylene selectivity from 80% to 87% and stability almost threefold. The enhanced selectivity and stability of PtB/SiO2 is most presumably due to favored desorption of propylene and mitigating coke formation under reaction conditions, respectively.« less

CO₂-assisted oxidative dehydrogenation of propane (CO₂-ODH) represents an attractive approach for propylene production and CO₂ utilization. As a soft oxidant, CO₂ can minimize overoxidation of the hydrocarbons to enhance the propylene selectivity while increasing the equilibrium yield. However, a major challenge of CO₂-ODH is the rapid deactivation of the catalysts. The current study focuses on designing Ce_xZr_1–xO₂-mixed oxide-supported CrO_x catalysts for CO₂-ODH with enhanced product selectivity and catalyst stability. By doping 0–30% Ce in the Ce_xZr_1–xO₂ mixed oxide support, propane conversion of 53–79% was achieved at 600 °C, with propylene selectivity up to 82%. Compared to the pure ZrO₂-supported catalystmore » (i.e., 5 wt %Cr/ZrO₂), 20–30 %Ce doped catalysts (i.e., 5 wt %Cr/Ce_0.2Zr_0.8O₂ and 5 wt %Cr/Ce_0.3Zr_0.7O₂) inhibited the formation of CH₄ and ethylene and improved propylene selectivity from 57 to 77–82%. Detailed characterizations of the 5%Cr/Ce_0.2Zr_0.8O₂ catalyst and density functional theory (DFT) calculations indicated that Cr³⁺ is the active species during the CO₂-ODH reaction, and the reaction follows a non-redox dehydrogenation pathway. Coke formation was determined to be the primary reason for catalyst deactivation, and the addition of Ce to the ZrO₂ support greatly enhanced the coke resistance, leading to superior stability. Furthermore, coke removal by oxidizing the catalyst in air is effective in restoring its activity.« less

Indium-containing chabazite zeolites show better stability and C ₃ H ₆ selectivity for propane dehydrogenation than In ₂ O ₃ , In/SiO ₂ and In/Al ₂ O ₃ . Extra-framework In ⁺ is identified as the stable active site upon reduction of an impregnated sample.

A series of supported 3% MoOx catalysts were synthesized by incipient-wetness impregnation of 5%-15% TaO_x surface modified γ-Al₂O₃ support. The catalysts were characterized by in situ spectroscopies (DRIFTS, Raman, UV-vis, XAS) and multiple chemical probes (C₂H₄/C₄H₈ titration, C₃H₆-TPSR, steady state propylene metathesis, NH₃-IR adsorption). The supported tantalum oxide phase was present as surface TaO_x sites on the γ-Al₂O₃ support that capped that Al₂O₃ surface hydroxyls. The change in available surface hydroxyls caused the subsequent anchoring of MoOx species to occur at different surface hydroxyls. This shifted the anchoring of MoO_x species from basic (Al-OH) to neutral (Al₂-OH) to more acidicmore » (Al₃-OH) surface hydroxyls as well as perturbation of the remaining alumina surface hydroxyls by the surface TaO_x sites. The TaO_x surface modified γ-Al₂O₃ support increased the number of activated surface MoO_x sites (Ns) by ~6x and the TOF by ~10x resulting in an increased activity of ~60x. In conclusion, It was found that the specific anchoring surface hydroxyls rather than the extent of oligomerization of the surface MoO_x sites control the number of activated MoO_x sites and TOF for propylene metathesis. No relationship between the nature of the surface Lewis/Brønsted acid sites and Ns and TOF were found to be present.« less