Search

We have used operando X-ray diffraction computed tomography (XRD-CT) analysed by rigorous crystallographic methods to reveal the behaviour of a zeolite/alumina based catalyst extrudate in the methanol-to-hydrocarbon reaction under working conditions (Treaction = 440 °C; ambient total pressure; WHSV = 12 gMeOH gextrudate−1 h−1). By doing so, we demonstrate that it is possible to precisely monitor how the cylindrical extrudate is deactivated during the process by gradual filling with coke from the outer surface towards the core. The operando data thus collected are simulated by a model taking both the reaction kinetics and the diffusion within the extrudate into account, and this analysis provides the tools required for future optimization of the degree of utilization of the catalyst. Both the operando experiment and post mortem XRD-CT of fully deactivated extrudates from a test reactor show clearly that the core of the catalyst body, around 1/3 of its volume, remains completely untouched by the reaction, even when the reactor output indicates zero activity.

Hydrogen is essential in a variety of large-scale chemical processes. As a carbon-free energy carrier, hydrogen has a potential for wide use within power production and transportation. However, most of the recent production methods involve the release of CO2 as a by-product. Hence, decarbonization of hydrogen production is one step to reduce CO2 emission into the Earth's atmosphere. Several process schemes have been suggested for low-carbon emission production of hydrogen. In this work, we show how to improve solid sorbents for the sorption-enhanced water-gas shift (SEWGS) process, which is a process that exploits a solid sorbent in the water-gas shift reactor to capture CO2 in situ and drive the process toward an improved hydrogen yield. We report herein a series of Co(x)Mg(3-)xAl materials based on hydrotalcites, promoted with various loadings of K. The materials have been characterized by BET, XRD, and NMR and tested for their CO2 adsorption performance in three adsorption-desorption cycles in a lab-scale fixed-bed reactor (20-22 bar, CO2 + steam as reactant gas, and isothermal conditions at 375 and 400 degrees C). The most promising material was subjected to a long-term test (120 adsorption-desorption cycles at similar conditions). This test indicates that a K-promoted Co1.5Mg1.5Al (22 wt % of added K2CO2 to the oxide) material has a higher cyclic capacity for CO2 than standard reference cases. We have estimated that the volumetric capacity (in mol/L unit) of this sorbent will be 23-26% higher than a standard reference material at 400 degrees C and 30-39% higher at 375 degrees C. This would, in fixed-bed columns, lead to significant reduction in the needed column volumes in the final process and reduce costs.

Silicoaluminophosphates (SAPOs) are a special class of zeolites that, due to their acidic and shape-selective properties, play a major role in ion exchange and separation processes and in crude oil cracking. SAPO-37 has the faujasite (FAU) topology same as zeolites X and Y, which are involved in more than 40% of the total crude oil conversion worldwide. A critical parameter that promotes detrimental structural transformations in SAPOs during real-life applications is the presence of humidity. In this study, we employ a multidisciplinary approach combining in situ synchrotron radiation powder X-ray diffraction (SR-PXRD), water adsorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations to describe the mechanism and reveal the reasons why SAPO-37 collapses upon contact with humidity below 345 K. SR-PXRD revealed that the sodalite (SOD) cages (subunits of the FAU structure) have the strongest affinity to water during hydration below 345 K. Furthermore, below 345 K, the faujasite framework takes up an order of magnitude more water molecules than at temperatures above 345 K. DRIFTS confirmed the presence of Si−OH and P−OH surface structural defects that act as hydration centers, accelerating the loss of a long-range order. Finally, DFT calculations showed that the enthalpy of water adsorption in the sodalite cage and the faujasite supercage is −212 and −13 kJ/mol, respectively. The results presented in this work are highly topical for understanding the effect of water on the frameworks of the SAPO microporous catalysts family. The notorious instability of SAPO-37 is the result of the accumulative contribution of topological, physical, and chemical effects, leading to an array of rapidly evolving cascading effects. Our work shows how advancements in SR-PXRD methodology and hardware give new insight into highly dynamic features previously difficult to observe. In addition, this work introduces the conceptual insight that nonhomogeneous sorption of molecular species will induce dynamic features with dramatic consequences at both molecular and atomic levels. This is a highly impactful factor opening research paths for further work within catalysis, porous material design and chemistry, and sorption reactions and processes.

Design and development of novel inorganic nanocarriers for encapsulation of natural antimicrobial substances for food packaging applications have received great interest during the last years. Natural nanoclays are the most investigated nanocarriers and recently interest has also grown in the synthetically produced porous silica particles. However, these different carrier matrices have not been compared in terms of their loading capability and subsequent release. In this study, the feasibility of porous silica particles (with different pore structures and/or surface functionalities) and commercially available nanoclays were evaluated as encapsulation matrices. Two well-studied antimicrobial substances, thymol and curcumin, were chosen as volatile and non-volatile model compounds, respectively. The encapsulation efficiency, and the subsequent dispersibility and release, of these substances differed significantly among the nanocarriers. Encapsulation of the volatile compound highly depends on the inner surface area, i.e., the protective pore environment, and an optimal nanocarrier can protect the encapsulated thymol from volatilization. For the non-volatile compound, only the release rate and dispersibility are affected by the pore structure. Further, water-activated release of the volatile compound was demonstrated and exhibited good antimicrobial efficacy in the vapor phase against Staphylococcus aureus. This comparative study can provide a base for selecting the right nanocarrier aimed at a specific food packaging application. No nanocarrier can be considered as a universally applicable one.

Coke formation is a major reason in deactivation of acidic zeolite catalysts in industrial processes such as methanol to hydrocarbons conversion. Protecting the surface of acidic zeolite with an inert porous shell can greatly hinder the coke formation on the surface, and hence boost the lifetime of the catalyst. In this work, a solid-state steam-assisted method for synthesis of such optimized protective shell (silicate-1, ~15 nm thickness) is designed. This general and simple protocol can be applied to acidic zeolite catalysts to improve their catalytic lifetime. The silicalite-1 shell is synthesized on mesoporous ZSM-5 zeolite to explore its catalytic activity in methanol to hydrocarbons conversion. XPS and TEM analysis confirm the coverage of mesoporous zeolite crystals by non-acidic shell. In addition, nitrogen physisorption shows the accessibility of mesoporous ZSM-5 via microporous silicalite-1 network. Applying this protective shell increases the lifetime of the catalyst by 100% and its conversion capacity by 130%, in comparison to mesoporous ZSM-5 without the shell. The controlled formation of thin layer of microporous silicalite-1 around mesoporous ZSM-5 crystals (without growth of individual silicalite-1) accounts for enhanced catalytic improvement.

The sorbent enhanced water-gas shift (SEWGS) process has great potential for CO2 capture and is thereby a realistic alternative to conventional post-combustion capture by gas emission scrubbing. Nevertheless, better performing materials are required to make SEWGS competitive against state-of-the-art scrubbing technologies. Dawsonite, NaAl(OH)2CO3, has a high carbonate content and forms in situ during CO2 capture on alkali metal promoted alumina, thereby showing potential in variable temperature capture processes. For the isothermal SEWGS process the performance in pressure swing absorption is essential. We evaluate here synthetic Dawsonites [MAl(OH)2CO3, M = K, Na] as CO2 sorbents for the SEWGS process based on data from in situ X-ray diffraction, combined thermogravimetric analysis and mass spectrometry, magic-angle spinning nuclear magnetic resonance spectroscopy, as well as high pressure fixed bed reactor tests at isothermal conditions. The results show that synthetic Dawsonites are promising sorbents for the SEWGS process, however, with moderate cyclic CO2 capacities (∼0.3–0.7 mmol CO2/g sorbent) in comparison to alkali-metal promoted hydrotalcites. On the other hand, the materials exhibit relatively fast sorption rates at low temperatures (280–310 °C), favoring the water-gas shift reaction in the SEWGS process.

In this study, hybrid mesoporous organosilicas (HMOs) were synthesized by using tetraethyl orthosilicate (TEOS) as the silica source and 1,4-bis(triethoxysilyl)benzene (BTB) in various ratios of BTB to TEOS. The two extreme cases of 0 and 100 mol% BTB were compared with the partial addition of BTB (25 mol%) and the partial absence of TEOS (75 mol% BTB). The synthesized mesoporous materials were characterized by means of powder X-ray diffraction (PXD), scanning electron microscopy (SEM) and helium pycnometry for the determination of skeletal density. The Brunauer–Emmett–Teller (BET) method was used for the determination of the specific surface area (SSA) and non-local density functional theory (NLDFT) calculations were employed for the determination of the pore size distribution (PSD). The hydrogen and methane sorption properties were investigated using a Sieverts apparatus under isothermal sorption equilibrium conditions at cryogenic and close to ambient temperatures, respectively. For hydrogen, the combination of phenyl rings with pores at the micro/mesopore border resulted in an increase in sorption capacity. The simultaneous presence of two different precursors increased the surface inhomogeneity, which led to a wider distribution of adsorption sites close to the micro/mesopore border, which favored the hydrogen sorption properties. The presence of the phenyl rings doubled the number of methane molecules that the material surface could accommodate. The partial substitution of TEOS by BTB (25 mol%) gave the same density of adsorbed methane as the non-hybrid material, which consisted of 100% BTB. The materials exhibited excellent reversibility and sorption stability upon aging. Their sorption performance was evaluated using the Toth model and was correlated with their structural characteristics. The fraction of micropores among the total number of pores was quantitatively correlated with the maximum storage capacity and the adsorbate–adsorbent interaction strength. Finally, for a low coverage of methane the enthalpy of adsorption was calculated by the Clausius–Clapeyron equation.

In order to mitigate climate change driven by the observed high levels of carbon dioxide (CO2) in the atmosphere, many micro and nano-porous materials are being investigated for CO2 selectivity, capture and storage (CCS) purposes, including zeolites, metal organic frameworks (MOFs), functionalized polymers, activated carbons and nano-silicate clay minerals. Key properties include availability, non-toxicity, low cost, stability, energy of adsorption/desorption, sorbent regeneration, sorption kinetics and CO2 storage capacity. Here, we address the crucial point of the volumetric capture and storage capacity for CO2 in a low cost material which is natural, non-toxic, and stable. We show that the nano-silicate Nickel Fluorohectorite is able to capture 0.79 metric tons of CO2 per m3 of host material - one of the highest capacities ever achieved - and we compare volumetric and gravimetric capacity of the best CO2 sorbent materials reported to date. Our results suggest that the high capture capacity of this fluorohectorite clay is strongly coupled to the type and valence of the interlayer cation (here Ni2+) and the high charge density, which is almost twice that of montmorillonite, resulting in the highest reported CO2 uptake among clay minerals.

Pre-combustion CO 2 capture technologies are becoming viable alternatives to more conventional post-combustion capture by gas emissions scrubbing. The sorbent enhanced water-gas shift (SEWGS) process is a promising future technology for CO 2 capture. However, the process needs better performing materials than those available today to be competitive against state-of-the-art scrubbing technologies. Layered double hydroxides (LDH) are a promising class of materials to improve the performance of the SEWGS process. These materials have a general formula of M 2+ 1−x M 3+ x (OH) 2 (A n− ) x/n ·mH 2 O, and can be tuned by substituting the metal species, changing the M 2+ /M 3+ ratio, adjusting the synthesis parameters to influence morphology or by adding so-called promotors to improve performance. To aid an ongoing systematic study looking at several of these parameters we have developed a simple yet efficient way of screening materials for further in-depth studies. The method is highly suitable for a typical laboratory setting, and is based on thermogravimetric analysis combined with cyclic exposure to selected gases. In this article we present the results of applying the method to a selection of benchmark materials.

Novel hexagonal Periodic Mesoporous Organosilicas (PMOs) and Disordered Mesoporous Organosilicas (DMOs) were synthesized by hydrolysis of 1,4-bis(trialkoxylsilyl) benzene precursor in alkaline aqueous solutions of different alkyl-trimethyl ammonium cations and evaluated for their hydrogen storage capacity. The PMO materials exhibit regular hexagonal pore arrangement and specific surface area between 640 and 782 m2 g−1 whereas the DMO materials have specific surface area that lies between 650 and 910 m2 g−1. The storage capacity of the materials is discussed in terms of number of molecules per surface unit. The materials exhibit a reversible hydrogen excess surface adsorption capacity up to 2.10 wt% at 6 MPa and 77 K. DFT calculations were performed to define the binding strength of hydrogen with the pore walls indicated an interaction energy value of −0.55 Kcal mol−1, higher than the interaction energy value of hydrogen with a single benzene or a benzene incorporated in the IRMOR-1 walls. Grand Canonical Monte Carlo (GCMC) simulations showed that no hydrogen molecule can be inserted inside the wall structure of the materials.

A good material for CO2 capture should possess some specific properties: (i) a large effective surface area with good adsorption capacity, (ii) selectivity for CO2, (iii) regeneration capacity with minimum energy input, allowing reutilization of the material for CO2 adsorption and (iv) low cost and high environmental friendliness. Smectite clays are layered nanoporous materials that may be good candidates in this context. Here we report experiments which show that gaseous CO2 intercalates into the interlayer nano-space of smectite clay (synthetic fluorohectorite) at conditions close to ambient. The rate of intercalation, as well as the retention ability of CO2 was found to be strongly dependent on the type of the interlayer cation, which in the present case is Li+, Na+ or Ni2+. Interestingly, we observe that the smectite Li-fluorohectorite is able to retain CO2 up to a temperature of 35°C at ambient pressure and that the captured CO2 can be released by heating above this temperature. Our estimates indicate that smectite clays, even with the standard cations analyzed here, can capture an amount of CO2 comparable to other materials studied in this context.

NaBH4 has long been identified as a viable hydrogen-storage material due to a theoretical gravimetric H2 capacity of 10.6 wt %. Because of the high enthalpy of decomposition of 108 ± 3 kJ mol–1, thermal decomposition of the pristine material does not occur until at least 500 °C, and thus NaBH4 has yet to be utilized in hydrogen-storage processes. In this study, NaBH4 has been milled with a variety of Ni-containing additives to investigate the effects on the temperatures required for thermal desorption of H2 by temperature-programmed desorption (TPD) measurements and the products characterized by powder X-ray diffraction (PXD). Ni-containing additives have been determined to significantly enhance the thermal desorption of H2 by at least 60 °C (Ni (65 wt %) on Si/Al2O3). PCT cycling experiments have been conducted to ascertain their effects on the reversible hydrogenation of the milled NaBH4. PXD analysis indicates that Ni reacts with B evolved during thermal decomposition to form NixBy species including Ni...