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The role of spacer length in macrocyclization reactions under confinement

(2024) Nandeshwar, Muneshwar; Weisser, Kilian; Ziegler, Felix; Frey, Wolfgang; Buchmeiser, Michael R.

We studied the influence of the distance of olefin metathesis catalysts from the inner surface of a mesoporous support on macrocyclization and Z‐selectivity under confinement. For these purposes, the cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) catalysts [Mo(N‐(2‐tBu‐C6H4)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)Br+ B(ArF)4-] Mo2, [Mo(N‐(2‐tBu‐C6H4)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)OTf+ B(ArF)4-] Mo3, [Mo(N‐(2,6‐Me2‐C6H3)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)Br+ B(ArF)4-] Mo5, and [Mo(N‐(2,6‐iPr2‐C6H3)(1‐mesityl‐3‐(3‐trimethoxysilylprop‐1‐yl)‐imidazol‐2‐ylidene)(CHCMe2Ph)(MeCN)+Br B(ArF)4-] Mo7 (B(ArF)4 = tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate), all containing a trimethoxysilylpropyl tether, were selectively immobilized inside the mesopores of SBA‐15. Under confinement, both macro(mono)cyclization (MMC) and Z‐selectivity were higher than in solution but lower than with catalysts directly bound to the surface of the mesoporous supports. These findings are in agreement with existing theoretical models on substrate and product distribution in mesopores, which suggest that the highest substrate concentration is found at the pore wall and that it increases with decreasing pore diameter.

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Open Access

Rayleigh invariance allows the estimation of effective CO2 fluxes due to convective dissolution into water‐filled fractures

(2025) Keim, Leon; Class, Holger

Convective dissolution of CO2 is a well‐known mechanism in geological storage of CO2. It is triggered by gravitational instability which leads to the onset of free convection. The phenomenon is well studied in porous media, such as saline aquifers, and the literature provides substantial evidence that onset times and effective flux rates can be estimated based on a characterization of instabilities that uses the Darcy velocity. This work extends the study of convective dissolution to open water‐filled fractures, where non‐Darcy regimes govern the induced flow processes. Numerical simulations using a Navier‐Stokes model with fluid density dependent on dissolved CO2 concentration were used to compute scenario‐specific results for effective CO2 entry rates into an idealized fracture with varying aperture, temperature, and CO2 concentration at the gas‐water interface. The results were analyzed in terms of dimensionless quantities. They revealed a Rayleigh invariance of the effective CO2 flux after the complete formation of a quasi‐stationary velocity profile, that is, after a certain entry length. Hence, this invariance can be exploited to estimate the effective CO2 entry rates, which can then be used, in perspective, in upscaled models. We have studied convective CO2 dissolution for two different fracture settings; the first one relates to karstification scenarios, where CO2 is the dominant driving force, and were stagnant‐water conditions in fractures have not yet received attention to date. The second setting is inspired from geological CO2 storage, where the literature provides only studies on convective CO2 dissolution for porous‐media flow with Darcy regimes.

Item

Open Access

Enhanced oxidation resistance of Pt‐containing Inconel 718 alloy through facilitated formation of protective chromia

(2024) Zheng, Jianshu; Zhang, Boning; Wang, Guowei; Liu, Lan; Shen, Chao; Song, Zhuorui; Zheng, Lei

This study investigates the influence of Pt addition on the oxidation behavior of a Cr2O3‐forming superalloy. Inconel 718 (IN718) alloys with varying Pt content were prepared and subjected to isothermal oxidation tests. The results demonstrate that Pt significantly enhances the oxidation resistance of IN718, as evidenced by reduced weight gain, thinner oxide layers, and smaller oxide particles. Pt addition also increases the activation energy for both initial interface oxidation and ion diffusion during long‐term oxidation. Furthermore, Pt promotes the formation of a Cr2O3 layer while suppressing the formation of other undesirable oxides, resulting in a more cohesive and stable oxide layer. The improved oxidation resistance is attributed to two key factors: during the initial oxidation stage, Pt, as a noble element, reduces the activity of the primary oxide‐forming element Cr to oxidative environments, thereby lowering its susceptibility to initial oxidation at the metal-oxidant interface. During long‐term oxidation, Pt preferentially substitutes for Ni in major phases such as γ‐Ni(Cr,Fe) and γ′‐Ni3(Al,Ti), locally increasing the Cr composition. This promotes Cr oxidation, effectively suppressing the oxidation of Ni or Fe. These findings suggest that Pt addition is a promising approach for enhancing oxidation resistance in alloy design.