Research activities and recent developments in the area of three-dimensional zeolites and their two-dimensional analogues are reviewed. Zeolites are the most important industrial heterogeneous ...catalysts with numerous applications. However, they suffer from limited pore sizes not allowing penetration of sterically demanding molecules to their channel systems and to active sites. We briefly highlight here the synthesis, properties and catalytic potential of three-dimensional zeolites followed by a discussion of hierarchical zeolites combining micro- and mesoporosity. The final part is devoted to two-dimensional analogues developed recently. Novel bottom-up and top-down synthetic approaches for two-dimensional zeolites, their properties, and catalytic performances are thoroughly discussed in this review.
Research activities and recent developments in the area of three-dimensional zeolites and their two-dimensional analogues are reviewed.
Great interest has arisen in the past years in the development of hierarchical zeolites, having at least two levels of porosities. Hierarchical zeolites show an enhanced accessibility, leading to ...improved catalytic activity in reactions suffering from steric and/or diffusional limitations. Moreover, the secondary porosity offers an ideal space for the deposition of additional active phases and for functionalization with organic moieties. However, the secondary surface represents a discontinuity of the crystalline framework, with a low connectivity and a high concentration of silanols. Consequently, hierarchical zeolites exhibit a less "zeolitic behaviour" than conventional ones in terms of acidity, hydrophobic/hydrophilic character, confinement effects, shape-selectivity and hydrothermal stability. Nevertheless, this secondary surface is far from being amorphous, which provides hierarchical zeolites with a set of novel features. A wide variety of innovative strategies have been developed for generating a secondary porosity in zeolites. In the present review, the different synthetic routes leading to hierarchical zeolites have been classified into five categories: removal of framework atoms, surfactant-assisted procedures, hard-templating, zeolitization of preformed solids and organosilane-based methods. Significant advances have been achieved recently in several of these alternatives. These include desilication, due to its versatility, dual templating with polyquaternary ammonium surfactants and framework reorganization by treatment with surfactant-containing basic solutions. In the last two cases, the materials so prepared show both mesoscopic ordering and zeolitic lattice planes. Likewise, interesting results have been obtained with the incorporation of different types of organosilanes into the zeolite crystallization gels, taking advantage of their high affinity for silicate and aluminosilicate species. Crystallization of organofunctionalized species favours the formation of organic-inorganic composites that, upon calcination, are transformed into hierarchical zeolites. However, in spite of this impressive progress in novel strategies for the preparation of hierarchical zeolites, significant challenges are still ahead. The overall one is the development of methods that are versatile in terms of zeolite structures and compositions, capable of tuning the secondary porosity properties, and being scaled up in a cost-effective way. Recent works have demonstrated that it is possible to scale-up easily the synthesis of hierarchical zeolites by desilication. Economic aspects may become a significant bottleneck for the commercial application of hierarchical zeolites since most of the synthesis strategies so far developed imply the use of more expensive procedures and reagents compared to conventional zeolites. Nevertheless, the use of hierarchical zeolites as efficient catalysts for the production of high value-added compounds could greatly compensate these increased manufacturing costs.
The present review is aimed at exploring the field of the catalytic cracking of polyolefins over solid acids, focusing on the role played by the catalysts toward the synthesis of fuels and chemicals ...as well as on the reaction systems currently used. Initially, conventional solid acids, such as micrometer sized crystal zeolites and silica–alumina, were used to establish the relationship among their activity, selectivity, and deactivation in the polyolefin cracking and the inherent properties of the catalysts (acidity, pore structure); however, the occurrence of steric and diffusional hindrances for entering the zeolite micropores posed by the bulky nature of the polyolefins highlighted the importance of having easily accessible acid sites, either through mesopores or by a high external surface area. This fact led toward the investigation of mesoporous materials (Al-MCM-41, Al-SBA-15) and nanozeolites, which allowed increasing the catalytic activities, especially for the case of polypropylene. Further advances have come by the application of hierarchical zeolites whose bimodal micropore–mesopore size distribution has turned them into the most active catalysts for polymer cracking. In this regard, hierarchical zeolites may be regarded as a clear breakthrough, and it is expected that future research on them will bring new achievements in the field of catalytic cracking of polyolefins. In addition, other materials with high accessibility toward the active sites, such as extra-large pore zeolites, delaminated zeolites, or pillared zeolite nanosheets, can also be considered potentially promising catalysts. From a commercial point of view, two-step processes seem to be the most feasible option, including a combination of thermal treatments with subsequent catalytic conversion and reforming, which allows the catalytic activity to be preserved against different types of deactivation.
Decomposition of CH
4 (natural gas) is one of the alternatives under study to achieve the sustainable production of hydrogen. No CO
2 or other greenhouse gases emissions are produced in this route ...and carbon is obtained as a solid co-product at the end of the reaction (
CH
4
⇆
C
+
2
H
2
). This process can be thermally or catalytically conducted and recent studies have demonstrated that the carbon obtained in the reaction can also show catalytic activity.
In this work, thermal and autocatalytic decomposition of methane were studied and compared with the steam reforming with and without CO
2 capture and storage from an environmental point of view, using life cycle assessment (LCA) tools. As well, different energetic scenarios were included in the study.
The selected functional unit was 1
Nm
3 of hydrogen and the LCA was focused on material and raw materials acquisition and manufacturing stages. The assessment was carried out with SimaPro 7.1 software by using Eco-indicator 95 method. Results showed that autocatalytic decomposition is the most environmental-friendly process for hydrogen production since presented the lowest total environmental impact and CO
2 emissions. Also, steam reforming with CO
2 capture and storage led to lower CO
2 emissions but higher total environmental impact than conventional steam reforming.
To characterise and compare the sociodemographic profiles and the frequency of common comorbidities for adults with chronic migraine (CM) and episodic migraine (EM) in a large population-based ...sample.
The American Migraine Prevalence and Prevention (AMPP) study is a longitudinal, population-based, survey. Data from the 2005 survey were analysed to assess differences in sociodemographic profiles and rates of common comorbidities between two groups of respondents: CM (ICHD-2 defined migraine; > or =15 days of headache per month) and EM (ICHD-2 defined migraine; 0-14 days of headache per month). Categories of comorbid conditions included psychiatric, respiratory, cardiovascular, pain and 'other' such as obesity and diabetes.
Of 24 000 headache sufferers surveyed in 2005, 655 respondents had CM, and 11 249 respondents had EM. Compared with EM, respondents with CM had stastically significant lower levels of household income, were less likely to be employed full time and were more likely to be occupationally disabled. Those with CM were approximately twice as likely to have depression, anxiety and chronic pain. Respiratory disorders including asthma, bronchitis and chronic obstructive pulmonary disease, and cardiac risk factors including hypertension, diabetes, high cholesterol and obesity, were also significantly more likely to be reported by those with CM.
Sociodemographic and comorbidity profiles of the CM population differ from the EM population on multiple dimensions, suggesting that CM and EM differ in important ways other than headache frequency.
Nanoscale systems that coherently couple to light and possess spins offer key capabilities for quantum technologies. However, an outstanding challenge is to preserve properties, and especially ...optical and spin coherence lifetimes, at the nanoscale. Here, we report optically controlled nuclear spins with long coherence lifetimes (T
) in rare-earth-doped nanoparticles. We detect spins echoes and measure a spin coherence lifetime of 2.9 ± 0.3 ms at 5 K under an external magnetic field of 9 mT, a T
value comparable to those obtained in bulk rare-earth crystals. Moreover, we achieve spin T
extension using all-optical spin dynamical decoupling and observe high fidelity between excitation and echo phases. Rare-earth-doped nanoparticles are thus the only nano-material in which optically controlled spins with millisecond coherence lifetimes have been reported. These results open the way to providing quantum light-atom-spin interfaces with long storage time within hybrid architectures.
A series of transition metal phosphides (Ni sub(2)P, Co sub(2)P and MoP) have been synthesized by temperature programmed reduction of the corresponding metal phosphate precursors loaded over ...mesostructured Al-SBA-15, mesoporous gamma -Al sub(2)O sub(3) (m-Al sub(2)O sub(3)) and ordered mesoporous carbon (CMK-3). Both the dispersion and metal phosphide phases attained are strongly influenced by the support features, such as their acidic and textural properties. XRD, TEM and H sub(2) chemisorption results revealed that the MoP phase was probably formed but it underwent a fast re-oxidation in air. On the other hand, metal phosphide formation was hindered over m-Al sub(2)O sub(3) as only metallic Ni and Co were detected. All the materials prepared have been evaluated as catalysts in hydrodeoxygenation (HDO) using phenol as a bio-oil model compound. The highest phenol conversions were attained with the catalysts based on the acidic supports (Al-SBA-15 and m-Al sub(2)O sub(3)). Nevertheless, Co sub(2)P/Al-SBA-15, Ni sub(2)P/m-Al sub(2)O sub(3) and Co sub(2)P/m-Al sub(2)O sub(3) yielded cyclohexanol as the main product denoting very low HDO efficiency. In contrast, Ni sub(2)P/Al-SBA-15 showed remarkable catalytic properties, being the only catalyst that provided almost full phenol conversion and extremely high HDO efficiency, with cyclohexane selectivity higher than 90%. This may be due to a synergetic effect between the high electron deficiency, generated by the Ni alpha + (0 < alpha < 1) species owing to an electron transfer from Ni to P and the different acidic sites present in the catalyst.
Feedstock recycling of plastic waste by thermal and catalytic processes is a promising route to eliminate this refuse (which is harmful to the environment) by obtaining, at the same time, products ...that are useful as fuels or chemicals. During the past decade, this option has undergone an important evolution from a promising scientific idea to an alternative that is very close to reality with commercial opportunities. Thus, several commercial processes have been developed worldwide, most of them especially addressed toward the preparation of diesel fuel. The present review highlights the most remarkable achievements of the field, providing a fundamental insight into this fascinating area and highlighting the main milestones that should be achieved in the next future for this alternative to become applied commercially on a large scale.
The present work focuses on the pathways through which catalytic pyrolysis of biomass into bio-oil proceeds and the effect of the operation conditions on parameters like bio-oil oxygen composition ...and mass yield, and also additional indicators, such as the distribution of both the oxygen and the chemical energy contained in the initial biomass among the different products. Acid washed wheat straw was used as biomass feedstock. The pyrolysis tests were performed in a lab-scale downdraft fixed-bed reactor working at atmospheric pressure, employing a nanocrystalline H-ZSM-5 zeolite as catalyst. A systematic study was carried out by decoupling both the thermal and catalytic reactions in order to evaluate the influence of three key variables: temperature of the thermal zone; temperature of the subsequent catalytic step and catalyst/biomass ratio. Increasing the pyrolysis temperature in the thermal zone resulted in more bio-oil* (bio-oil in water-free basis) production to the detriment of char and water fractions. In contrast, a significant reduction in bio-oil* fraction, due to decarbonylation and decarboxylation, occurred when increasing the catalytic bed temperature from 400 up to 500 °C. A similar effect was observed by varying the catalyst/biomass ratio since it increased the production of CO, CO
2
, light olefins and coke at the expense of a decline in the bio-oil* yield. Nevertheless, this bio-oil contains oxygen amounting to as low as 10 wt%, while retaining about 38% of the energy yield. Char, coke and gaseous hydrocarbons contain a great part of the biomass chemical energy, hence their formation should be suppressed or minimized to further improve the bio-oil* energy yield. At high catalyst/biomass ratios, the bio-oil becomes rich in aromatic compounds, both oxygenated and hydrocarbons, while the amount of sugars, furans, carboxylic acids, and other oxygenated products is strongly reduced.
The present work systematically studies the effect of the operation conditions of biomass catalytic pyrolysis on parameters like bio-oil oxygen content and mass yield, but also on additional indicators, such as the distribution of oxygen and chemical energy among the products.
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A process where power and biomass are converted to Fischer-Tropsch liquid fuels (PBtL) is compared to a conventional Biomass-to-Liquid (BtL) process concept. Based on detailed process ...models, it is demonstrated that the carbon efficiency of a conventional Biomass to Liquid process can be increased from 38 to more than 90% by adding hydrogen from renewable energy sources. This means that the amount of fuel can be increased by a factor of 2.4 with the same amount of biomass. Electrical power is applied to split water/steam at high temperature over solid oxide electrolysis cells (SOEC). This technology is selected because part of the required energy can be replaced by available heat. The required electrical power for the extra production is estimated to be 11.6 kWh per liter syncrude (C5+). By operating the SOEC iso-thermally close to 850 °C the electric energy may be reduced to 9.5 kWh per liter, which is close to the energy density of jet fuel. A techno-economic analysis is performed where the total investments and operating costs are compared for the BtL and PBtL. With an electrical power price of 0.05 $/kWh and with SOEC investment cost of the 1000 $/kW(el), the levelized cost of producing advanced biofuel with the PBtL concept is 1.7 $/liter, which is approximately 30% lower than for the conventional BtL. Converting excess renewable electric power to advanced biofuel in a PBtL plant is a sensible way of storing energy as a fuel with a relatively high energy density.
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