Water Adsorption Applications Research Articles

Monolithic Zirconium-based Metal-Organic Frameworks for Energy-Efficient Water Adsorption Applications.

Space cooling and heating, ventilation and air conditioning (HVAC) accounts for roughly 10% of global electricity use and is responsible for ca. 1.13 gigatonnes of CO2 emissions annually. Adsorbent-based HVAC technologies have long been touted as an energy-efficient alternative to traditional refrigeration systems. However, thus far, no suitable adsorbents have been developed which overcome the drawbacks associated with traditional sorbent materials such as silica gels and zeolites. Metal-organic frameworks (MOFs) offer order of magnitude improvements in water adsorption and regeneration energy requirements. However, the deployment of MOFs in HVAC applications has been hampered by issues related to MOF powder processing. Herein, we develop three high-density, shaped, monolithic MOFs (UiO-66, UiO-66-NH2 and Zr-Fumarate) with exceptional volumetric gas/vapor uptake - solving previous issues in MOF-HVAC deployment. We visualized the monolithic structures across the mesoporous range using SAXS and lattice-gas models, giving accurate predictions of adsorption characteristics of the monolithic materials. We also demonstrate that a fragile MOF such as Zr-fumarate can be synthesized in monolithic form with a bulk density of 0.76 gcm-3 without losing any adsorption performance, having a coefficient of performance (COP) of 0.71 with a low regeneration temperature (≤ 100 ᵒC). This article is protected by copyright. All rights reserved.

Comparative evaluation of MIL-101(Cr)/calcium alginate composite beads as potential adsorbents for removing water vapor from air

Recently, MIL-101(Cr) as a porous coordination polymer is of great interestin water adsorption due to its water stability and excellent water uptake capacity. However, its microcrystalline nature puts limitations on its practical applications. Therefore, achieving an efficient method to produce the desired shaped forms while preserving the intrinsic properties of the powder has been a challenge. In this study, the MIL-101(Cr) powder material was produced in >300 g quantities with an overall yield of around 65% and an average SBET = 3101 m2 g−1 and it was shaped into spherical beads with mean size of 2.1 ± 0.2 mm by using sodium alginate and calcium chloride solution through an ionotropic gelation technique. The water vapor adsorption and the breakthrough behavior of prepared adsorbent were investigated in comparison with commercial adsorbents. In addition, hydrothermal stability, optimized regeneration temperature and mechanical strength of the adsorbent beads were examined. The results showed that at 25 °C and 1 bar the shaped adsorbent affording of water uptake of 1.1 g/g at 75% relative humidity, which is 290, 250 and 350 % more than activated alumina, 13X and 3A molecular sieves, respectively. Furthermore, it showed a breakthrough time as high as that for activated alumina and 3A molecular sieve. The complete regeneration was achieved at as low as 80 °C and the adsorbent showed stable performance after 20 consecutive water adsorption/desorption cycles. The outcome of this study suggests the high potential of the ionotropic gelation to shape MIL-101(Cr) powder material into bead form and use in water adsorption applications.

Improving Computational Assessment of Porous Materials for Water Adsorption Applications via Flat Histogram Methods

Water adsorption in porous materials has recently drawn considerable attention for its tremendous potential in environmental applications such as water harvesting from air. A key step to the deployment of such technologies is the selection of optimal adsorbent materials as material performance directly impacts their efficiency. Owing to the vast materials space, computational studies may play a critically important role in facilitating the selection of the best material. The current state-of-the-art method to predict adsorption behavior of porous materials, the grand canonical Monte Carlo (GCMC) simulations, however, can converge very slowly due to its inefficient sampling of the phase space accessible to the system and may yield unreliable results. To this end, we have demonstrated a method from a class of techniques known as flat histogram methods, which can sample the accessible states of the system much more efficiently. Further, a so-called C-map method is proposed herein to efficiently determine the applicability of a material in water adsorption applications. These methods can offer unprecedented insights into the behaviors of the GCMC-predicted isotherms and, more importantly, are promising for large-scale computational screening of porous materials for water adsorption.

Atomistic insight in the flexibility and heat transport properties of the stimuli-responsive metal-organic framework MIL-53(Al) for water-adsorption applications using molecular simulations.

To exploit the full potential of metal-organic frameworks as solid adsorbents in water-adsorption applications, many challenges remain to be solved. A more fundamental insight into the properties of the host material and the influence that water exerts on them can be obtained by performing molecular simulations. In this work, the prototypical flexible MIL-53(Al) framework is modelled using advanced molecular dynamics simulations. For different water loadings, the presence of water is shown to affect the relative stability of MIL-53(Al), triggering a phase transition from the narrow-pore to the large-pore phase at the highest considered loading. Furthermore, the effect of confinement on the structural organisation of the water molecules is also examined for different pore volumes of MIL-53(Al). For the framework itself, we focus on the thermal conductivity, as this property plays a decisive role in the efficiency of adsorption-based technologies, due to the energy-intensive adsorption and desorption cycles. To this end, the heat transfer characteristics of both phases of MIL-53(Al) are studied, demonstrating a strong directional dependence for the thermal conductivity.

Air-Con Metal–Organic Frameworks in Binder Composites for Water Adsorption Heat Transformation Systems

Metal–organic frameworks (MOFs) currently receive high interest for cycling water adsorption applications like adsorption heat transformation for air-conditioning purposes. For practical use in ads...

Detailed investigation on properties of novel commercial mesoporous silica materials

A detailed investigation on the main characteristics was conducted on a novel and unique group of industrially produced mesoporous silica material. Six materials from a TMPS group were selected in the respect to their pore size ranging from 1.8 nm up to 4.2 nm. Four of the selected samples were also made with silica doping making them potentially attractive for water adsorption applications with the advantage of the pore-tuning possibilities for specific use. The surface area, pore volume and pore size distribution of these materials were established by standard nitrogen adsorption at T = 77.4 K showing typical values of mesoporous silica materials such as high surface area in the range from 600 to 750 m 2 g −1 and pore volumes reaching 0.38 cm 3 g −1 in the case of the smallest TMPS-1.5A and 0.98 cm 3 g −1 in the case of the largest TMPS-4R. The wall thickness was found out regular around 1.5 ~ 2 nm with lower values attributed to bigger pore sized samples promising good structural integrity. Specific heat showed regular values through all the samples at ~ 0.85 J g −1 K −1 . Additional information on the structural characteristics and the effect on the amount of silanol group presence and the difference on regular and aluminium-doped samples were studied by means of 29 Si DD/MAS NMR. Additionally, elemental analysis by EDS was conducted as well. All the characteristics of the TMPS materials were compared to the standard mesoporous silica materials SBA-15 and MCM-41. • Bulk produced aluminium-doped mesoporous silica materials. • Variable pore size from 1.5 nm to 7 nm, typically. • High thermal stability and low pore size distribution. • Typical hexagonal structure and spherical particle cells size of 50–200 nm. • High water adsorption and hydrophilic properties of Al-doped samples.

Heat properties of a hydrophilic carboxylate-based MOF for water adsorption applications

The development of new porous materials is expanding the boundaries of applications related to gas adsorption, including gas separation, catalysis, drying and energy storage. The specific heat capacity and thermal conductivity of the adsorbent materials are important parameters in the engineering process. For instance, in the adsorptive heat pump technology, the energy generation and transfer by the adsorbent during the adsorption and desorption process of the adsorbate directly determines the energy efficiency of the working cycle. However, the thermal properties’ data for the novel types of adsorbent materials, metal–organic frameworks (MOF), is often lacking in the literature. This work followed a protocol particularly relevant for the measurement of MOFs’ thermal properties under powder form. A very promising hybrid material for energy storage, separation and other applications, MIL-160, has been tested as a reference. The specific heat capacity of the material was measured at temperatures ranging from 20 °C to 75 °C with a heat flow type differential scanning calorimeter (DSC). The thermal conductivity was measured with a transient heating source technique within a similar temperature range. By fitting the experimental data to the Clausius-Clapeyron model, the performance of this material in the application of adsorptive heat pump was obtained.

Influence of salt on nanozeolite-Y particles size synthesized under organic template-free condition

Abstract Zeolite-Y is one of the preferred micro-sized zeolite commonly used as a catalyst and for adsorption and separation. The size and shape of zeolite crystals have great influence on their properties therefore the synthesis of nano-size zeolite-Y has received much attention with straightforward methodology. In the present study, nano-sized zeolite-Y crystals were synthesized by the clear-solution (CS) and dense-gel (DG) methods in the presence of alkali metal chloride salt (i.e. NaCl) under organic template-free condition. The influence of the salt on the growth of nano zeolite-Y crystal size and morphology was studied systematically by varying the NaCl concentration. Depending on the method selected for synthesis (CS or DG), the growth of nano-size crystals was affected by the addition of NaCl content. In the CS method, the size of zeolite-Y crystals was found to decrease from 120 nm to 60 nm with an increase of salt content which signify the influence of salt in hindering diffusion of nutrients to the crystal nuclei in a homogeneous starting gel. Whereas, the spherical aggregates of well-shaped nanocrystals were formed with an increase of crystal size in the case of DG method. This is attributed to the suppression of the rate of crystal dissolution in dense-gel system upon introduction of salt ions. The nano-size zeolite-Y synthesized by both methods exhibited comparatively higher water adsorption capacity (15.4 ± 0.38–16.2 ± 0.41 mmol/g) than commercially available micro-size zeolite-Y (12.9 mmol/g) which can be attributed to the contribution of the meso/macropores arising from the inter-crystal voids. Hence, the synthesized nano-size zeolite-Y can be considered as promising adsorbents for water adsorption applications.

Synthesis of nanoporous zeolite-Y and zeolite-Y/GO nanocomposite using polyelectrolyte functionalized graphene oxide

Zeolite-Y with different particle size was synthesized and the growth of crystals was studied in the presence of a cationic polyelectrolyte (PDDA) functionalized graphene oxide (PDDA-GO). The assembly of PDDA-GO was found to be cooperative in the growth of pure zeolite-Y crystals with various particle sizes (400 nm and 100 nm). The submicron size zeolite-Y particles were grown with entrapment of the GO nanosheets and the complete removal of PDDA-GO by calcination under air atmosphere assisted nanoporosity generation in the zeolite crystals. Whereas the growth of nano-size zeolite-Y particles occurred through both side of GO surface and produce nanocomposite with inter-particles porosity. The synthesized nanoporous zeolite-Y and zeolite-Y/GO nanocomposite showed higher water adsorption capacity (259–291 mg/g) than microporous zeolite-Y that placed these adsorbents as promising candidate for water adsorption applications.

Role of Structural Defects in the Water Adsorption Properties of MOF-801

The nanoporous and tunable nature of metal–organic frameworks (MOFs) has made them promising adsorbents for water adsorption applications such as water harvesting and adsorptive heat pumps. In these applications, water adsorption properties in MOFs play a crucial role. However, understanding their structural defects and how defects influence adsorption thermodynamics remains limited to date. In this work, by employing Monte Carlo techniques and first-principle density functional theory calculations, we investigate the effect of defects on the water adsorption properties in MOF-801 structures at an atomic level. Our calculations show that the adsorption isotherm in perfect MOF-801 (without defects) greatly deviates from that measured experimentally. With the introduction of defects with a high density, a reasonably good agreement can be achieved, suggesting that a high defect density in MOF-801 may be responsible for its hydrophilic adsorptive behaviors. Further, water adsorption properties in MOF-801 stru...

Simple process for preparing mesoporous sol-gel silica adsorbents with high water adsorption capacities

In this work we present a simple route to prepare sol-gel silica samples for water adsorption applications. These materials were not chemically modified neither heat-treated before performing the water adsorption tests at 30 °C and atmospheric pressure. The materials studied in this work were examined by N2 adsorption and Fourier transform infrared spectroscopy (FTIR). The samples prepared herein showed higher adsorption capacities than a commercial silica commonly used as water adsorbent. It was shown that it is possible to produce materials with high water adsorption capacities controlling the sol pH. Samples obtained from sols with very low pH presented high number of small pores, whereas those prepared from sols without acid displayed larger pore; both cases increase the volume of pores and, consequently, the water uptake of the material. It is an important finding since several works available in the literature deals with complex and multi-step procedures, commonly using pore-formers or pore-expanders agents, which makes them cost ineffective and time consuming. The absence of acidic reactants in the preparation step of the adsorbent with the highest adsorption capacity represents an advantage in terms of safety and environmental sustainability, besides reinforcing the simplicity of the processing route employed herein. The pseudo-second order kinetic model was the best model to fit the water adsorption kinetics of the investigated samples, indicating a high interaction between the silica surface and water molecules. Besides, the water desorption was successfully carried out in relatively short times (<2 h) and at temperatures as low as 90 °C.

Organosilanes functionalization of alumino-silica zeolites for water adsorption applications

Standard zeolites 4A and 13X have been synthesized and modified by reaction with dimethoxydimethyl silane, propyltrimethoxy silane, octyltrimethoxy silane and octadecyltrimethoxy silane, respectively. The modified zeolites were analyzed by X-Ray diffractometry, FTIR spectrometry and water adsorption measurements. Results showed that silane molecules bonded to the external surface of zeolite particles with no chance of entering the zeolite porosity. Only “short-chain” silanes dimethoxydimethyl and propyltrimethoxy grafting demonstrated having an influence on the hydrophilic behavior of the modified zeolites. The main effect was a shift of the adsorption isotherms in the low partial pressure range towards a more hydrophobic trend, which represents an interesting result for adsorption application when low temperature regenerations are required.

Characterization of metal-organic frameworks by water adsorption

The water physisorption properties and the water stability of the metal-organic frameworks HKUST-1 (=(Cu 3(BTC) 2) (BTC = benzene-1,3,5-tricarboxylate)), ZIF-8, MIL-101, MIL-100(Fe) and DUT-4 (=Al(OH)(NDC)) (NDC = naphthalene-2,6-dicarboxylate) were studied. The water physisorption isotherms were compared to nitrogen physisorption isotherms and the chemical stability after water adsorption was investigated. Water adsorption does not only provide information about specific surface area, pore size and pore volume, but can also be used to estimate hydrophobicity and stability towards moisture. Both HKUST-1 and DUT-4 turned out to be unstable in direct contact with water, whereas the MIL-materials and ZIF-8 do show stability. The highest water adsorption affinity was observed for HKUST-1. Even though unstable in liquid water, for applications in trace water removal or molecular sensing HKUST-1 is a promising material. ZIF-8 is highly inert but hydrophobic. The MIL-materials are both likely candidates for water adsorption applications, but higher concentrations of the adsorbate are needed to attain an effective adsorption. DUT-4 shows insufficient adsorption capacity, but stability at ambient conditions is given.