Detection of the delayed condensation effect and determination of its impact on the accuracy of gas adsorption pore size distributions

Macroscopic, highly disordered, mesoporous materials present a continuing challenge for accurate pore structure characterization. The typical macroscopic variation in local average pore space descriptors means that methods capable of delivering statistically representative characterizations are required. Gas adsorption is a representative but indirect method, normally requiring assumptions about the correct model for data analysis. In this work we present a novel method to both expand the range, and obtain greater accuracy, for the information obtained from the main boundary adsorption isotherms by using a combination of data obtained for two adsorptives, namely nitrogen and argon, both before and after mercury porosimetry. The method makes use of the fact that nitrogen and argon apparently ‘see’ a different pore geometry following mercury entrapment, with argon, relatively, ‘ignoring’ new metal surfaces produced by mercury porosimetry. The new method permits the study of network and pore–pore co-operative effects during adsorption that substantially affect the accuracy of the characteristic parameters, such as modal pore size, obtained for disordered materials. These effects have been explicitly quantified, for a typical sol-gel silica catalyst support material as a case study. The technique allowed the large discrepancies between modal pore sizes obtained from standard gas adsorption and mercury thermoporometry methods to be attributed to the network-based delayed condensation effect, rather than spinodal adsorption. Once the network-based delayed condensation effect had been accounted for, the simple cylindrical pore model and macroscopic thermodynamic Kelvin-Cohan equation were then found sufficient to accurately describe adsorption in the material studied, rather than needing a more complex microscopic theory. Hence, for disordered mesoporous solids, a proper account of inter-pore interactions is more important than that of intra-pore adsorbate density distribution, to obtain accurate pore size distributions

Understanding the spatial distribution of coke deposition within bimodal micro-/mesoporous catalysts using a novel sorption method in combination with pulsed-gradient spin-echo NMR

AbstractA new method for the determination of the spatial distribution of metal surface area within bimodal micro-/mesoporous solids has been developed. This novel technique involves incorporating a nonane pre-adsorption stage between two successive chemisorption experiments. This method has been used to probe the distribution of platinum amongst the micropores and mesopores of a range of bi-functional PtH-MFI catalysts, each possessing differing surface acidities, which have been used for benzene alkylation with ethane. It has been found that the catalyst with the lowest Si/Al ratio, and thus highest number of acid sites, also possessed the largest metal surface area within its microporosity. This catalyst was also the one that deactivated most rapidly, with coke being deposited predominantly within the micropore network. This was attributed to the bi-functional mechanism for coke formation at higher temperatures. Pulsed-gradient spin-echo NMR has also been used to show that a combination of higher mesopore platinum concentration and higher mass transport rates facilitated greater coke deposition within the mesoporosity

Hyperpolarised xenon MRI and time-resolved X-ray computed tomography studies of structure-transport relationships in hierarchical porous media

© 2020 Elsevier B.V. Catalysed diesel particulate filter (DPF) monoliths are hierarchical porous solids, as demonstrated by mercury porosimetry. Establishing structure-transport relationships, including assessing the general accessibility of the catalyst, is challenging, and, thus, a comprehensive approach is necessary. Contributions, from each porosity level, to transport have been established using hyperpolarised (hp) xenon-129 magnetic resonance imaging (MRI) of gas dispersion within DPF monoliths at variable water saturation, since X-ray Computerised-Tomography, and 1H and 2H NMR methods, have shown that porosity levels dry out progressively. At high saturation, hp 129Xe MRI showed gas transport between the channels of the monolith is predominantly taking place at channel wall intersections with high macroporosity. The walls themselves make a relatively small contribution to through transport due to the distribution of the micro-/meso-porous washcoat layer away from intersections. Only at low saturation, when the smallest pores are opened, do hp 129Xe MR images became strongly affected by relaxation. This observation indicates accessibility of paramagnetic (catalytic) centres for gases arises only once the smallest pores are open

An ether-free, internally coordinated dialkylcalcium(II) complex

The alkylpotassium compound KC{SiMe3}(2){SiMe(2)hpp} (hppH = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine) reacts with a slurry of CaI2 in toluene to give the internally coordinated dialkylcalcium(II) complex Ca(C{SiMe3}(2){SiMe(2)hpp})(2). The molecular structure in the solid state shows short Ca-N and long Ca-C bonds and a wide C-Ca-C angle. The H-1 and C-13 NMR spectra suggest that the metallacycle is transiently opened in toluene at room temperature

Mechanistic studies on the reductive cyclooligomerisation of CO by U(III) mixed sandwich complexes; the molecular structure of [(U(eta-C8H6{Si(i)Pr3-1,4}2)(eta-Cp*)]2(mu-eta1:eta1-C2O2)

The stoichiometric reaction of 1 equiv of CO with [(U(eta-C8H6{SiiPr3-1,4}2)(eta-Cp*)] affords the linear diuranium ynediolate complex [(U(eta-C8H6{SiiPr3-1,4}2)(eta-Cp*)]2(mu-eta1:eta1-C2O2) which does not react with further CO to give the deltate derivative [(U(eta-C8H6{SiiPr3-1,4}2)(eta-Cp*)]2(mu-eta1:eta2-C3O3). Spectroscopic and computational studies suggest a plausible mechanism for the formation of the deltate complex, in which a "zig-zag" diuranium ynediolate species is the key intermediate

Operando neutron scattering – following reactions in real time using neutrons

The complexation of NiCl2 with 2,2'-Bipyridine was followed using quasielastic neutron scattering to observe reaction progress. Water adsorption in chabazite with time resolution was observed using strain induced in the aluminosilicate framework with a high-resolution engineering diffractometer. These reactions illustrate the recent progress and possibilities in using neutron probes to observe realistic catalytic reactions as they progress

Insight into molecular behaviour in microporous catalysis using quasielastic neutron scattering

The potential of quasielastic neutron scattering (QENS) to give a unique and detailed insight into the behaviour of active species in both established and emerging microporous catalytic systems is illustrated in 3 case studies, studying the methanol-to-gasoline (MTG) and ammonia selective catalytic reduction (NH -SCR) processes, and the zeotype catalysed Beckmann rearrangement.We analyse the dynamics of methanol in ZSM-5 catalysts both with and without the hydrocarbon pool, resulting from the MTG reaction taking place at 623K and 673 K, to determine the effects of catalyst use on molecular mobility. Using QENS, we observe that methanol is immobile on the instrumental time scale in the fresh catalyst (ZSM-5-F) and in the sample used to convert methanol for 3 days at 623 K (ZSM-5-623). However, in zeolite ZSM-5-673 (MTG at 673 K for 3 days) we observe isotropic methanol rotation. The observed differences between the zeolites in methanol dynamics are attributed to the development of mesoporosity in ZSM-5-673 due to the high reaction temperature of 673 K, leading to dislodgement of lattice Al as is evident from NMR data.In collaboration with Johnson Matthey, QENS and MD were combined and applied to the Cu-CHA zeolite catalysed NH3-SCR process. The effect of counterion presence on ammonia mobility was studied through comparing the diffusivity in a Cu-CHA sample and a counterion free sample. Our studies of the effect of Cupresence in CHA found little impact of the counterion on NH diffusivity, with the NH performing jumps through the 8-rings between cages at the same rate between both samples. The finding was rationalized by MD simulations which observed that coordination shells formed around the Cu , shielding other NH molecules which were mobile, allowing the unimpeded intercage jump diffusion.In the context of the zeotype catalysed Beckmann rearrangement, we study the mobility of cyclohexanone oxime in a microporous catalyst in comparison with a hierarchical micro/mesoporous analogue.Couterintuitively, we find that the introduction or larger mesopores into the catayst structure lowers the overall mobility of the oxime due to the presence of a larger immobile fraction. The increase in the immobile fraction is explained by the walls of the mesopore being terminated by strongly adsorbing terminal silanol groups, immobilising the oxime at the low loadings studied, the interaction of which has been observed by neutron vibrational spectroscopy