98 Matching Annotations
  1. May 2019
    1. which enables maximum regeneration with minimal temperature increase

      This means that the water desorption process is highly efficient, not requiring a large amount of energy from heat.

    2. By considering both the adsorption and desorption dynamics, a porosity of 0.7 was predicted to yield the largest quantity of water.

      The results of the simulations show that if the porosity is too low, the adsoprtion process becomes slower. In contrast, if the porosity is too high, more MOF is required to harvest the same amount of water but the process is slower. The simulations showed that the ideal porosity was 0.7.

    3. We also report a device based on this MOF that can harvest and deliver water (2.8 liters of water per kilogram of MOF per day at 20% RH) under a nonconcentrated solar flux less than 1 sun (1 kW m–2), requiring no additional power input for producing water at ambient temperature outdoors.

      In this study, the authors demonstrate that a MOF-based device can harvest water from the atmosphere at humidity and temperature conditions similar to a desert, without requiring any more energy than natural sunlight. Since this paper was published, the authors published another paper with the results of a devise test in the Arizona desert. In the "Related Content" tab, see their 2018 Nature Communications paper.

  2. Apr 2019
    1. M. Eddaoudi et al., Science 295, 469–472 (2002)

      A systematic study on the influence of the organic linkers on the pore properties of a metal-organic framework. In this work, the authors use the same metallic cluster as a base and demonstrate how the small molecule linkers can be functionnalized to tune its properties.

    2. an order of magnitude

      About 10 times. Let's say zeolites (a conventional adsorbent) could adsorb 2 liters, then MOF-801 could adsorb an order of magnitude greater—that is, 20 liters.

    3. noncondensables

      Gases that cannot easily be turned into liquids. In this case, noncondensables refers to the gases present in air which are mainly dioxygen, dinitrogen, and argon.

    4. global horizontal irradiation

      The amount of solar light received by a horizontal surface on Earth.

    5. solar absorptance of 0.91

      The fraction of the sun radiation that penetrates the surface. A solar absorptance of 0.91 means that 91% of the sun's radiation penetrates the material and is converted to heat.

    6. Experiments were performed in a RH-controlled environmental chamber interfaced with a solar simulator.

      To test the material in a laboratory setup, the authors use an enclosed chamber in which conditions such as humidity, temperature, and solar illumination can be regulated. This guarantees control over the experimental conditions and reproducibility.

    7. activated (solvent removal from the pores) by heating at 150°C under vacuum for 24 hours

      Heating under reduced pressure lowers the boiling point of liquids. This allows all the solvent and water molecules trapped in the MOF to evaporate easily, emptying all the cavities before starting the experiment.

    8. face-centered cubic topology

      Describes the 3D geometry of the MOF. MOF-801 is constituted of a repetition of cubes having one cluster in each corner and each center of the faces. Figure 1B shows one of these small cubes constituting the repeating unit of the MOF.

    9. In particular, MOF-801 is suitable for regions where RH is merely 20% (e.g., North Africa),

      MOF-801 was selected for this study because it adsorbs a large amount of water even when a small amount of water is present in the atmosphere, as is often the case in arid regions such as deserts.

    10. diurnal

      During the day. It is the opposite of nocturnal, which means during the night.

    11. The amount of water that can be harvested with MOFs can be much greater than with dew-harvesting systems, which become impractical at RHs less than 50%

      To date, the existing systems for water-harvesting cannot be efficiently used in environments with low relative humidity levels.

    12. porosity

      The ratio of the cavities' volume to the total volume of the material. MOFs are made of microscopic crystals, which are responsible for the presence of these cavities. Porosity can range from 0 to 1, a porosity of 0 meaning that there are no cavities within the structure.

    13. e.g., zeolites and silica gels)

      Zeolites and silica gels are minerals made mainly of aluminum and silica. They also have porous structures which allow them to trap molecules within their cavities. This property makes them useful as adsorbants for molecules such as water.

    14. Porous materials, such as zeolites, silica gels, and MOFs, can harvest water from air by adsorption over a wide range of humidity values

      It has been found that a variety of porous materials can trap water from air in small cavities within its structures. This capacity is dependent on the material. For the water harvesting process to be practically usable, the material needs to be able to absorb a large amount of water even when only a small concentration of water is present in the atmosphere (low relative humidity).

    15. microcrystalline

      Made of atoms and molecules arranged in a highly ordered microscopic structure that extends in every direction.

    16. dewing (3–6) from moist air and fog capture

      There has been a lot of interest in developing a system capable of harvesting water from air and fog. However, systems developed so far either require a very large amount of water in the air (high humidity), which is not suitable for dry regions, or need a large amount of energy to allow condensation of the water. These systems are therefore neither usable in dry conditions nor economically viable. In the "Related Content" tab, check out a response the paper authors wrote that addresses these issues.

    17. 1 sun

      Unit of light intensity corresponding to 1000 watts per square meter. For reference, if you stood one meter away from a standard 60-watt light bulb, the light intensity would be equal to about 4.8 watts per square meter.

    18. [Zr6O4(OH)4(fumarate)6]

      The chemical composition of MOF-801. This specific MOF is based on zirconium (Zr) metal ions linked together by fumarate molecules, which are small organic compounds made of carbon, oxygen, and hydrogen.

  3. Mar 2019
    1. We performed a parametric study, including varying the packing porosity (0.5, 0.7, and 0.9) and layer thickness (1, 3, 5, and 10 mm), and determined the time and amount of harvestable water for a solar flux of 1 sun

      The authors examined different parameters to optimize the water harvesting properties of the material. The porosity and thickness of the material are assumed to be the parameters having the largest effect because they determine the amount of water that can be adsorbed for a chosen compacted adsorbent layer.

    2. recycling

      In the case of MOF, recycling describes the ability to release the water from the pores multiple times.

    3. stability

      The capacity of the MOF to go through multiple cycles of adsorption-desorption of water without losing water uptake capacity.

  4. Dec 2018
  5. Oct 2018
    1. To fully utilize the steep step in water uptake in the MOF-801 isotherm, a temperature difference of ~45°C between the condenser and the layer is necessary to achieve desorption at 10% RH

      The efficiency of the water-harvesting relies on the temperature difference between the MOF and the condenser, as well as the relative humidity, for the condenser to be able to collect all the water vapor desorbed from the MOF. The authors found that at 10% RH, a difference of 45°C is needed to reach the maximum water-harvesting capability.

    2. By saturating the MOF layer with ambient air at a solar flux less than 1 sun, ~0.3 liters kg–1 potentially can be harvested.

      The results of experiments and simulations show that the device can, at the maximum, collect 0.3 liters of water per kg of MOF material in normal outdoor usage.

    3. Continuously harvesting water in a cyclic manner for a 24-hour period with low-grade heat at 1 kW m–2 can yield ~2.8 liters kg–1 day–1 or ~0.9 liters m–2 day–1with a 1-mm-thick layer

      The result of the simulation shows that undergoing several cycles of adsoprtion-desoprtion can produce almost 3 liters of water per 24 hours with 1 kg of the MOF material. This is the case for a 1 mm-thick layer.

    4. However, at higher porosities, a thicker MOF-801 layer is required to harvest a sufficient amount of water, but the time scale and transport resistance for intercrystalline diffusion also scales with the MOF layer thickness as t ~ Lc2/Dv, where, t, Dv, and Lc are the time scale, intercrystalline diffusivity, and characteristic length scale (i.e., layer thickness), respectively.

      Porosity is a key parameter in the performance of the MOF material. If the porosity is too low, the adsoprtion process becomes slower because the molecules have to diffuse through the crystal. In contrast, if the porosity is too high, more MOF is required to harvest the same amount of water and the process is also slower.

    5. Each water-harvesting cycle, ~0.24 liters kg–1 were harvested (Fig. 2D), as determined by integrating the water-harvesting rate.

      The water harvesting rate measures liters per kilogram per second and varies during the experiment. To obtain the total amount of water harvested, the authors integrated the water harvesting rate over the whole duration of the experiment (100 minutes). In this time, the MOF water-harvesting device collected the equivalent of a small glass of water from the surrounding air.

    6. parasitic


    7. passive

      The system does not require any additional source of energy.

    8. boundary conditions and idealizations in the simulation

      The authors define a series of constraints for their simulation. The scientific model assumes facts about the phenomenon that make the problem easier to solve. In this case, the authors assume among others that the pore structure is homogeneous, that the vapor pressure is the same for the MOF and the condenser, and that the diffusivity of the crystal does not vary.

    9. diffusion

      Spontaneous movement of a substance due to the difference in its concentration between different areas. For example, if you put a tea bag in water, the tea leaves will spread by diffusion, giving the water its color.

    10. sorption

      Refers to both adsorption and absorption, which are, respectively, the adhesion of a substance to a surface or its incorporation into another substance.

    11. isotherm

      A curve joining points measured at the same temperature. In this case, the water uptake capacity of the MOF is measured by varying either the humidity or the vapor pressure but keeping the temperature constant (isotherm) during the measurement.

    12. vapor diffusion resistance

      Value reflecting the fact that vapor diffusion between different materials is limited by different factors such as the accessibility of the pores.

    13. For visualization purposes, we used a condenser with a temperature controller to maintain the temperature slightly below ambient levels but above the dew point, in order to prevent vapor condensation on the inner walls of the enclosure.

      To simplify the experiments, the author maintain the condenser at a temperature cooler than the surrounding temperature. This results in the condenser being cooler than the walls of the box, so that all the water condenses on the condenser and can be easily collected. In practice, ambient temperature is enough to harvest water without the need for additional cooling of the condenser.

    14. thermoelectric cooler

      A device using electricity to transfer heat. Think of it as a refrigerator.

    15. A solar flux (1 kW m–2, air mass 1.5 spectrum) was introduced to the graphite-coated substrate layer

      The authors simulate natural sunlight in the laboratory. The air mass is the ratio of the path length that sunlight has to travel through the atmosphere to the vertical path length (when sun is at the highest point in the sky, which corresponds to the shortest path length). An air mass of 1.5 is the condition most commonly use to quantify the efficiency of solar devices.

    16. passive heat sink

      Heat sinks are devices that are used to transfer heat, usually to keep a hot component cool. Although active heat sinks like fans require an input of energy, passive heat sinks are usually made of aluminum fins and rely on their surface area to dissipate heat and do not require energy.

    17. UiO-66

      A different metal-organic-framework. The name UiO is used for the MOFs developed at the University of Oslo. This MOF is constituted of the same metal clusters as MOF-801 but with different linkers between them.

    18. incident solar flux

      The amount of sunlight that reaches the surface.

    19. The environmental temperature above standard ambient temperature was necessary to perform the experiments at >1 kPa

      In order to efficiently harvest water, a large difference of temperature between the MOF and the condenser is needed. In their experiment, the authors set the temperature at 35°C in order to be able to condense the vapor with a condenser at 10°C.

    20. dynamic

      Constantly changing. In this case, the MOF temperature varies during adsorption and desorption of water.

    21. exothermic

      That releases heat.

    22. adsorbate

      The substance adsorbed, in this case water.

    23. porous

      Containing holes within its structure.

    24. flux

      The amount that passes through a substance. In this case, it is the amount of sunlight received by a surface.

    25. Atmospheric water

      The water present in the atmosphere as vapor, mainly due to evaporation from the oceans.

    26. metal-organic framework

      Structure composed of metal ions linked together by small molecules. This structure repeats itself to form a network that is often porous and can be used to trap chemicals in their cavities like a sponge.

    27. S. H. Schneider, Encyclopedia of Climate and Weather (Oxford Univ. Press, 1996

      A comprehensive description of the phenomena responsible for the world's climate and weather events, as well as the history of atmospheric science.

    28. Image of a water-harvesting prototype with activated MOF-801
    29. MIT

      Massachusetts Institute of Technology.

    30. In this simulation, MOF-801 was initially equilibrated at 20% RH, and the vapor content in the air-vapor mixture that surrounds the layer during desorption increased rapidly from 20 to 100% RH at 25°C

      The authors performed a simulation in conditions reflecting an accurate, real-life use of the MOF. The results show an increase in RH upon water desorption, followed by return to the initial 20% value upon water adsorption.

    31. heat transfer coefficient

      Quantitative measure of the flow of heat between two materials.

    32. the crystal diameter of MOF-801 is only ~0.6 μm

      The diameter of MOF-801 was determined by a method called scanning electron microscopy. The surface of the MOF is scanned with a beam of electrons which gives information about the surface.

      See figure S5 in the Supplementary Materials.

    33. A theoretical model was developed to optimize the design of the water-harvesting process with MOF-801, which was further validated with the experimental data.

      The authors developed a computational model to simulate the behavior of the MOF material. Through this approach, they can study the influence of the material parameters on the water harvesting properties and then confirm their result experimentally. Their goal is to screen a large number of parameters computationally to optimize the system.

    34. A powder of MOF-801 was synthesized as reported

      The preparation of MOF-801 was described by the same group in a previous study. The method consists of heating the organic component in a solvent with a reagent containing the metal ions. This forms the MOF as a solid that can then be filtered.

    35. We carried out the adsorption-desorption experiments for water harvesting with MOF-801 at 20% RH

      The author first quantified the water harvesting capacity of the MOF. For this purpose, they use 20% relative humidity for their measurements, as it is representative of the low humidity in dry regions of the world where an efficient water-harvesting method is most needed.

    36. good performance driven by aggregation of water molecules into clusters within the pores of the MOF

      The authors found that a key parameter in the water uptake capacity of this MOF is the geometry that the water molecules adopt inside the cavities, forming multiple hydrogen bonds. This phenomenon is explained principally by the size of the pores of the MOF.

    37. desorption

      Phenomenon through which a substance is released from a surface. In this case, water vapor released from an MOF. It is the opposite of adsorption.

    38. adsorption

      Phenomenon through which a substance adheres to a surface. In this case, water vapor adsorbed to the MOF. It is the opposite of desorption.

    39. Although MOFs have already been considered in numerous applications

      MOFs are porous materials that can be used to capture and release a variety of ions and small molecules.

      Read more and listen to a podcast in Chemistry World: https://www.chemistryworld.com/podcasts/mofs-metalorganic-frameworks-/3007204.article

    40. relative humidity (RH)

      The amount of moisture in the air compared to the maximum amount the air can hold at that same temperature.

    41. Two-thirds of the world’s population is experiencing water shortages

      Less than 1% of the total water on Earth is drinkable and easily accessible. Due to an uneven distribution, the world is facing a global water crisis, and billions of people do not have regular access to clean drinking water.

      Read more on National Geographic: https://news.nationalgeographic.com/2018/03/world-water-day-water-crisis-explained/

    42. MOF-801

      MOF stands for metal-organic framework. MOF-801 refers to the unique name used to identified this specific metal-organic framework.

    43. low humidity levels (down to 20%)

      In arid areas, where an efficient water-harvesting system is most needed, the relative humidity can go as low as about 20% with extremely large temperature variations.

      Read more on Sciencing: https://sciencing.com/humidity-mojave-desert-19526.html

    44. S. Narayanan, S. Yang, H. Kim, E. N. Wang, Int. J. Heat Mass Transfer 77, 288–300 (2014).

      A computational modeling study of adsorption dynamics evaluating how multiple parameters such as vapor pressure and porosity affect the adsorption performance. The simulation results were confirmed experimentally with an adsorbing zeolite.

    45. J. H. Cavka et al., J. Am. Chem. Soc. 130, 13850–13851 (2008).

      The design and characterization of a zirconium-based MOF displaying very high thermal and chemical stability.

    46. H. Furukawa, K. E. Cordova, M. O’Keeffe, O. M. Yaghi, Science 341, 1230444 (2013).

      A review on the preparation of MOFs and their structural properties as well as their applications.

    47. J. Lee et al., Chem. Soc. Rev. 38, 1450–1459 (2009).

      An overview of the usage of the use of MOFs as selective catalyst to perform chemical reactions between substrates.

    48. H. Furukawa et al., J. Am. Chem. Soc. 136, 4369–4381 (2014).

      The same group previously described how to prepare MOF-801 and characterized its water adsorption properties.

    49. R. V. Wahlgren, Water Res. 35, 1–22 (2001).

      A review on existing systems to extract water from the atmosphere.

    50. M. M. Mekonnen, A. Y. Hoekstra, Sci. Adv. 2, e1500323 (2016).

      A study showing that about 4 billion people do not have access to fresh water for at least a month per year, half of them in China and India.

    51. Therefore, to predict the prototype’s water-harvesting potential under equilibrium conditions, we extended the desorption time for the simulation

      The design of the prototype results in a slower desorption process than was considered in the initial calculations. The authors modified the parameters of their simulation to take this into account and have a model reflecting the experimental conditions.

    52. heat sink

      A device or substance that absorbs excessive heat.

    53. active cooling

      Using an external source of energy to cool a system, as opposed to passive cooling that does not require energy.

    54. Last, a proof-of-concept MOF-801 water-harvesting prototype was built to demonstrate the viability of this approach outdoors

      To demonstrate the utility of their material in real conditions, the authors built a small device containing the MOF and a condenser and tested it outdoors.

    55. intracrystalline diffusivity

      Measure of the capability of the material to allow a substance to pass through the crystal by diffusion.

    56. void spacing

      Distance between the cavities.

    57. Knudsen diffusion

      Model of diffusion occurring when molecules travel through very small pores.

    58. inter- and intracrystalline vapor diffusion

      How well vapor can pass between crystals and through a single crystal by diffusion.

    59. The model framework was based on mass and energy conservation, incorporating adsorption dynamics parameters

      The authors built their simulation model based on previous studies about adsorption processes defining the important parameters to optimize the adsorption process.

    60. COMSOL Multiphysics

      A software used to model and simulate scientific problems.

    61. equilibrated


    62. Thermocouples

      Sensors used to measure temperature.

    63. isobaric

      At constant pressure.

    64. abscissa

      The horizontal axis.

    65. packing porosity

      The porosity of the final material composed of the MOF in the copper foam.

    66. The powder was infiltrated into a porous copper foam with a thickness of 0.41 cm and porosity of ~0.95, which was brazed on a copper substrate to create an adsorbent layer

      The MOF is pressed onto a substrate made of copper metal in order to have a material that conducts heat efficiently and that can cover a large surface.

    67. clusters

      An ensemble of atoms or molecules linked together.

    68. condenser

      An apparatus made for turning vapor into liquid.

    69. yield

      In this case, the yield is the amount of water that can be harvested.

    70. The flexibility (20–22) with which MOFs can be made and modified at the molecular level

      MOFs are composed of metal ions connected by small organic molecules. Both these small building blocks can be varied to afford a large diversity of different MOFs with properties tailored for specific applications.

    71. catalysis

      One of the numerous applications of MOFs has been the ability to induce selective chemical reactions between substrates. The metal ions within the MOF enable the reaction to occur only for the substrates that are able to enter in the cavities.

    72. low-grade energy

      An energy that cannot be fully converted into another type of energy. Heat is considered a low-grade energy. Low-grade energies are naturally less expensive than high-grade energies such as electricity.

    73. The water in the form of vapor and droplets in the atmosphere, estimated to be about 13 thousand trillion liters

      Where is Earth's water? The water contained in the atmosphere represents about 0.001% of the total water on Earth, and about 3% of the accessible freshwater.

      Read more on USGS: https://water.usgs.gov/edu/earthwherewater.html