Zn-modified MCM-41 as support for Pt catalysts

The effect of Zn location and reduction temperature have been studied on pure and Zn-modified MCM-41 supported Pt catalysts. Experimental results show that incorporation of Zn into the siliceous framework of the MCM-41 material introduces structural oxidized Zn centres with a high stability towards a reduction treatment even at high temperatures. XP spectra confirm the high stability of these Zn species although, at the highest reduction temperature (723 K), a high mobility of the doping cations from structural positions to the inner wall of the mesopores is observed. Catalytic studies in the chemoselective hydrogenation of crotonaldehyde show that the presence of Zn produces an important decrease in the catalytic activity of the Pt nanoparticles, in accordance with Pt dispersion measurements. However, the presence of Zn improves the selectivity towards the formation of crotyl alcohol. Compared with a conventional impregnated PtZn/MCM-41 catalyst, structural Zn centres in the doped Pt/ZnMCM-41 catalysts exhibit a higher polarizing/activation of the carbonyl double bond giving rise to a higher yield to crotyl alcohol, the product of interest. Additionally, these Zn-modified MCM-41-based Pt catalysts show an excellent behaviour in an important industrial reaction such as the dehydrogenation of iso-butane to iso-butene, with a selectivity of 100%, which remains stable with time on stream.Es la versión preprint del artículo. Se puede consultar la versión final en https://doi.org/10.1016/j.apcata.2008.08.02

Unusual flexibility of mesophase pitch-derived carbon materials:an approach to the synthesis of graphene

Structural flexibility in a petroleum pitch-derived carbon material has been indirectly evaluated using X-ray diffraction (XRD), immersion calorimetry and inelastic neutron scattering (INS) measurements. Exposure of the carbon material to an organic solvent (e.g., n-nonane) gives rise to a large internal rearrangement, associated with a drastic re-ordering of the graphitic microdomains. These structural changes are also associated with a high flexibility of the internal porous network, as observed by inelastic neutron scattering measurements. The internal rearrangement and the structural flexibility could be responsible for the excellent performance of this kind of activated carbons in a wide variety of adsorption processes. Last but not least, the structural characteristics of these carbon materials composed of graphitic microdomains has been used to synthesize graphene “egg-like” flakes following a simple procedure based on exfoliation with organic solvents

Zr-Porphyrin Metal–Organic Framework as nanoreactor for boosting the formation of hydrogen clathrates

We report the first experimental evidence for rapid formation of hydrogen clathrates under mild pressure and temperature conditions within the cavities of a zirconium-metalloporphyrin framework, specifically PCN-222. PCN-222 has been selected for its 1D mesoporous channels, high water-stability, and proper hydrophilic behavior. Firstly, we optimize a microwave (MW)-assisted method for the synthesis of nanosized PCN-222 particles with precise structure control (exceptional homogeneity in morphology and crystalline phase purity), taking advantage of MW in terms of rapid/homogeneous heating, time and energy savings, as well as potential scalability of the synthetic method. Second, we explore the relevance of the large mesoporous 1D open channels within the PCN-222 to promote the nucleation and growth of confined hydrogen clathrates. Experimental results show that PCN-222 drives the nucleation process at a lower pressure than the bulk system (1.35 kbar vs 2 kbar), with fast kinetics (minutes), using pure water, and with a nearly complete water-to-hydrate conversion. Unfortunately, PCN-222 cannot withstand these high pressures, which lead to a significant alteration of the mesoporous structure while the microporous network remains mainly unchanged.Authors would like to acknowledge financial support from Ministerio de Ciencia e Innovación (Project PID2019-108453GB-C21 and PID2022-141034OB-C22), Consejo Superior de Investigaciones Científicas (CSIC) for internal funds (Intramural project, 202280I170), and Conselleria de Innovación, Universidades, Ciencia y Sociedad Digital (Project CIPROM/2021/022). A portion of this research used resources at the Spallation Neutron Source, a DOE Office of Science User Facility operated by the Oak Ridge National Laboratory (project IPTS-29742.1)

Gate-opening effect in ZIF-8: the first experimental proof using inelastic neutron scattering

The gate-opening phenomenon in ZIFs is of paramount importance to understand their behavior in industrial molecular separations. Here we show for the first time using in situ inelastic neutron scattering (INS) the swinging of the –CH3 groups and the imidazolate linkers in the prototypical ZIF-8 and ZIF-8@AC hybrid materials upon exposure to mild N2 pressure.The authors acknowledge financial support from MINECO projects: MAT2013-45008-p and CONCERT Project-NASEMS (PCIN-2013-057). EVRF gratefully acknowledge support from MINECO (Spain) for his Ramón y Cajal grant (RyC-2012-11427). DFJ thanks the Royal Society (UK) for funding through a University Research Fellowship and Dr Axel Zeitler for interesting discussions. This research benefited from the use of the VISION beamline (IPTS-13608) at ONRL’s Spallation Neutron Source and the VirtuES (Virtual Experiments in Spectroscopy) project, (LDRD 7739), which are supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy, under Contract No. DE-AC0500OR22725 with UT Battelle, LLC

Unusual flexibility of mesophase pitch-derived carbon materials:an approach to the synthesis of graphene

Structural flexibility in a petroleum pitch-derived carbon material has been indirectly evaluated using X-ray diffraction (XRD), immersion calorimetry and inelastic neutron scattering (INS) measurements. Exposure of the carbon material to an organic solvent (e.g., n-nonane) gives rise to a large internal rearrangement, associated with a drastic re-ordering of the graphitic microdomains. These structural changes are also associated with a high flexibility of the internal porous network, as observed by inelastic neutron scattering measurements. The internal rearrangement and the structural flexibility could be responsible for the excellent performance of this kind of activated carbons in a wide variety of adsorption processes. Last but not least, the structural characteristics of these carbon materials composed of graphitic microdomains has been used to synthesize graphene “egg-like” flakes following a simple procedure based on exfoliation with organic solvents

New insights into the breathing phenomenon in ZIF-4

Structural changes in ZIFs upon adsorption remain a paradigm due to the sensitivity of the adsorption mechanism to the nature of the organic ligands and gas probe molecules. Synchrotron X-ray diffraction under operando conditions clearly demonstrates for the first time that ZIF-4 exhibits a structural reorientation from a narrow-pore (np) to a new expanded-pore (ep) structure upon N2 adsorption, while it does not do so for CO2 adsorption. The existence of an expanded-pore structure of ZIF-4 has also been predicted by molecular simulations. In simulations the expanded structure was stabilized by entropy at high temperatures and by strong adsorption of N2 at low temperatures. These results are in perfect agreement with manometric adsorption measurements for N2 at 77 K that show the threshold pressure for breathing at ∼30 kPa. Inelastic neutron scattering (INS) measurements show that CO2 is also able to promote structural changes but, in this specific case, only at cryogenic temperatures (5 K).The authors would like to acknowledge financial support from the MINECO (MAT2016-80285-p), Generalitat Valenciana (PROMETEOII/2014/004), H2020 (MSCA-RISE-2016/NanoMed Project), Spanish ALBA synchrotron (Projects AV-2017021985 and IH-2018012591) and Oak Ridge beam time availability (Project IPTS-20843.1). JSA and JGL acknowledge financial support from UA (ACIE17-15) to cover all the expenses for INS measurements at Oak Ridge. JGL acknowledges GV (GRISOLIAP/2016/089) for the research contract

Rapid and efficient hydrogen clathrate hydrate formation in confined nanospace

Clathrate hydrates are crystalline solids characterized by their ability to accommodate large quantities of guest molecules. Although CH4 and CO2 are the traditional guests found in natural systems, incorporating smaller molecules (e.g., H2) is challenging due to the need to apply higher pressures to stabilize the hydrogen-bonded network. Another critical limitation of hydrates is the slow nucleation and growth kinetics. Here, we show that specially designed activated carbon materials can surpass these obstacles by acting as nanoreactors promoting the nucleation and growth of H2 hydrates. The confinement effects in the inner cavities promote the massive growth of hydrogen hydrates at moderate temperatures, using pure water, with extremely fast kinetics and much lower pressures than the bulk system.We would like to acknowledge financial support from Ministerio de Ciencia e Innovación (Project PID2019-108453GB-C21), MCIN/AEI/10.13039/501100011033 and EU “NextGeneration/PRTR” (Project PCI2020-111968 /3D-Photocat) – JSA. Neutron scattering experiments were performed at ORNL’s Spallation Neutron Source, IPTS-27062, supported by the Scientific User Facilities Division, Office of Basic Energy Sciences, US DOE, under Contract No. DE-AC0500OR22725 with UT Battelle, LLC—J.S.A., Y.Q.C., L.D., A.J.R.C.. We gratefully acknowledge research support from the Hydrogen Materials—Advanced Research Consortium (HyMARC), established as part of the Energy Materials Network under the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technology Office, under Contract Number DE-AC05-00OR22725—R.B.-X. This manuscript has been authored in part by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The publisher acknowledges the US government license to provide public access under the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan)

Control of the pore size distribution and its spatial homogeneity in particulate activated carbon

There are circumstances where it is desirable to achieve a particular, optimal, pore size distribution (PSD) in a carbon, including in the molecular sieving, gas storage, CO2-capture and electrochemical energy storage. Activation protocols that cycle a carbon a number of times between a low-temperature oxygen chemisorption process and a higher temperature pyrolysis process have been proposed as a means of yielding such desired PSDs. However, it is shown here that for PFA-based char particles of ∼100 μm in size, only the super-micropores are substantially developed under such an activation protocol, with the ultra-micropores being substantially un-touched. It is also shown that a typical CO2-activation process yields similar control over PSD development. As this process is nearly 15 times faster than the cyclic-O2 protocol and yields larger pore volumes and areas for a given level of conversion, it is to be preferred unless spatial homogeneous porosity within the particles is also desired. If such homogeneity is desired, it is shown here that CO2 activation should continue to be used but at a rate of around one-tenth the typical; this slow rate also has the advantage of producing pore volumes and areas substantially greater than those obtained using the other activation protocols.CH acknowledges a joint scholarship provided by China Scholarship Council (CSC) and the University of Adelaide. SS acknowledges the award of International Postgraduate Research Scholarship (IPRS) from the University of Adelaide. SHM acknowledges the award of a President’s Scholarship from the University of South Australia. The support of the Australian Research Council Discovery Program (DP110101293) is also gratefully acknowledged

Understanding the breathing phenomena in nano-ZIF-7 upon gas adsorption

Synchrotron X-ray diffraction and inelastic neutron scattering measurements have been applied to evaluate the breathing phenomena in small nanocrystals of ZIF-7 upon gas adsorption. The experimental results show that an extended solvent exchange process with methanol is crucial to get a solvent-free narrow pore structure. Under these conditions, nano-ZIF-7 is indeed able to adsorb N2 with a total BET surface area of around 380 m2 g−1, in close agreement with theoretical predictions. The breathing phenomenon upon nitrogen adsorption is accompanied by a phase-to-phase transition, from a narrow-pore (phase II) to a large-pore (phase I) structure and a suppression of the cooperative deformation of the framework involving mainly the flapping motion of the benzimidazolate (bIm) ligand with the 4- and 6-membered rings. Whereas nitrogen requires temperature and pressure conditions close to condensation (close to 1 bar and 77 K) to induce the breathing in ZIF-7, CO2 can do it under milder conditions (at room temperature and low relative pressures). These results reflect that the nature of the adsorptive probe and the gas–framework interactions, rather than the molecular diameter and/or shape, play a crucial role in defining the pressure and temperature conditions required to induce the breathing. The presence of two different cavities in ZIF-7 as suggested by theoretical predictions, one with a window diameter of below 0.4 nm (cavity A) and the other with a pore size of around 0.6 nm (cavity B), has been confirmed experimentally using immersion calorimetry.J. S. A. and C. C. C. acknowledge financial support from the University of Alicante (ACIE16-04) to cover all the expenses for the INS measurements at ORNL. J. S. A. gratefully acknowledges financial support from MINECO (MAT2016-80285-p), European Union H2020 (MSCA-RISE-2016/NanoMed Project) and Generalitat Valenciana (PROMETEOII/2014/004), Spanish ALBA synchrotron for beam time availability (Project ID: 2016021724) and Oak Ridge beam time availability (Project ID: IPTS-16291.1)

Tuning porosity in macroscopic monolithic metal-organic frameworks for exceptional natural gas storage.

Widespread access to greener energy is required in order to mitigate the effects of climate change. A significant barrier to cleaner natural gas usage lies in the safety/efficiency limitations of storage technology. Despite highly porous metal-organic frameworks (MOFs) demonstrating record-breaking gas-storage capacities, their conventionally powdered morphology renders them non-viable. Traditional powder shaping utilising high pressure or chemical binders collapses porosity or creates low-density structures with reduced volumetric adsorption capacity. Here, we report the engineering of one of the most stable MOFs, Zr-UiO-66, without applying pressure or binders. The process yields centimetre-sized monoliths, displaying high microporosity and bulk density. We report the inclusion of variable, narrow mesopore volumes to the monoliths' macrostructure and use this to optimise the pore-size distribution for gas uptake. The optimised mixed meso/microporous monoliths demonstrate Type II adsorption isotherms to achieve benchmark volumetric working capacities for methane and carbon dioxide. This represents a critical advance in the design of air-stable, conformed MOFs for commercial gas storage