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In-situ microwave hydrothermal synthesis and performance of chromium-substituted aluminophosphate zeolite coating on aluminum foil
Abstract
In this research, an in-situ microwave hydrothermal synthesis strategy for chromium-substituted aluminophosphate (CrAlPO-5) zeolite coating on aluminum foil was introduced. The influences of seed coating, molar ratio of Cr2O3/P2O5, crystallization temperature and time on the performance of adsorbent coating were investigated systematically. The crystal phase, composition and the morphology of the as-synthesized CrAlPO-5 adsorbents were characterized by X-ray diffractometer, Fourier infrared spectrometer and Scanning electron microscopy, and its adsorption and desorption performances were evaluated by water vapor adsorption and temperature programmed desorption. Results showed that CrAlPO-5 zeolite coating could be conveniently synthesized on aluminum foil by in-situ microwave hydrothermal method with the molar ratio of Cr2O3/P2O5 of 0.01–0.06, the temperature of 180–210 °C and within 15–40min. Compared with the CrAlPO-5 adsorbent prepared without seed coating, the one synthesized by seed-inducing method had a more complete crystal phase, higher relative crystallinity and larger specific surface area, and displayed a smaller and more perfect hexagonal prism crystal morphology as well as a thicker coating (100 μm). Moreover, it exhibited slightly higher saturation adsorption capacity. All results indicate that the novel synthesis technique of CrAlPO-5 zeolite coating on aluminum foil provides a guidance for the fabrication of desiccant coating on heat exchanger in ACH system.
... Fly ash, a solid waste with huge discharge, has been widely used for the synthesis of zeolite X owing to its high silicon and aluminum content [15][16][17]. Numerous studies have been established to synthesize zeolite X, such as one-step method [18,19], two-step method [20,21], microwave-assisted two-step method [22,23] and ultrasonic-assisted method [24]. Normally, a large amount of solvent involves water or organic solution is required in these processes, which inevitably results in wastewater pollution [25]. ...
Zeolite X is a potential electromagnetic wave (EMW) absorption material owning to its advantages of good dielectric properties, light weight and large specific surface area. Herein, high-purity zeolite X was prepared from fly ash using a green synthesis approach, in which only trace water was used and achieved zero discharge of waste water. Physiochemical properties of fabricated zeolite X were comprehensively evaluated through XRD, PSD, SEM, TG-DSC and BET test. XRD pattern shows the successful preparation of zeolite X with good crystallinity and high purity, and the average particle size is ~ 2.45 μm, which conforms to Gaussian distributions. The fabricated zeolite X exhibits typical octahedral structure and good thermal stability. It is noteworthy that the specific surface area is up to 473.56 m2/g, representing porous structure, which is beneficial to EMW attenuation. What’s more, the abundant crystal water and adsorbed water existing in the sample is conductive to dipole polarization, which further consume EMW energy. The effective absorption bandwidth (EAB) of the synthesized zeolite X is 2.08 GHz (13.36–15.44 GHz) at the thickness of 2.5 mm. This study provides an eco-friendly approach to change waste into valuables, and moreover the synthesized zeolite X is a potential EMW absorbing material.
... Numerous studies have been established to synthesize zeolite X, such as one-step method [18][19], twostep method [20][21], microwave-assisted two-step method [22][23] and ultrasonic-assisted method [24]. Normally, a large amount of solvent involves water or organic solution is required in these processes, which inevitably results in wastewater pollution [25]. ...
Zeolite X is a potential electromagnetic wave (EMW) absorption material owning to its advantages of good dielectric properties, light weight and large specific surface area. Herein, high-purity zeolite X was prepared from fly ash using a green synthesis approach, in which only trace water was used and achieved zero discharge of waste water. Physiochemical properties of fabricated zeolite X were comprehensively evaluated through XRD, PSD, SEM, TG-DSC and BET test. XRD pattern shows the successful preparation of zeolite X with good crystallinity and high purity, and the average particle size is ~2.45 μm, which conforms to Gaussian distributions. The fabricated zeolite X exhibits typical octahedral structure and good thermal stability. It is noteworthy that the specific surface area is up to 473.56 m 2 /g, representing porous structure, which is beneficial to EMW attenuation. What’s more, the abundant crystal water and adsorbed water existing in the sample is conductive to dipole polarization, which further consume EMW energy. This study provides an eco-friendly approach to change waste into valuables, and moreover the synthesized zeolite X is a potential EMW absorbing material.
... These features rendered them as potential sorbent material in air water-harvesting systems. A mixed system design (Tu and Hwang 2019;Tu et al. 2018;Vivekh et al. 2020) will open a new research field for studying newly developed sorbents, such as SAPO-34 (Zheng et al. 2015), CrAlPO-5 zeolite (Fang et al. 2020), Ag-FAPO-34 (Zheng et al. 2019), PVA-LiCl (Vivekh et al. 2019), and FAM-Z01 (Tsujiguchi et al. 2019;Vivekh et al. 2018) for air water-harvesting systems. ...
Air water-harvesting systems are considered important technologies for overcoming the global water scarcity issue. Various relative humidity and cost considerations make sorbent-based air water-harvesting systems the most desirable technologies among current air water-harvesting systems. The limited availability of commercial instruments for air–water harvesting systems indicates a lack of fundamental studies on this field. In this regard, this review paper discusses and presents progress in the fields of sorbent materials and condensation and system design and future considerations in accelerating the commercialization of these technologies. Particularly, bio-inspired composition and design, mix design systems, use of renewable energy sources, modification of available sorbent materials according to functionalization, and composites are factors that require further attention. Fundamental studies of sorbent stability and life cycle, water absorbency, adsorption kinetics, heat and mass transport, regeneration condition, water-collecting surface design, and system design are essential.
... They are essentially used in the formation of glassy-based systems that find excellent applications in optical, tissue engineering, dental implants, catalysts, and cores in the electrical appliances based on their electrical and magnetic properties [4][5][6][7]. ...
Amorpho-structural and magnetic analysis of Al 2 O 3-P 2 O 5-Na 2 O/NiO magnetic porous nanocomposites in the influence of SiO 2 as a nucleating agent is studied. Concurrently, the effect of the incorporated NiO nanoparticles on the structural, morphological, optical, and magnetic characteristics of the Al 2 O 3-P 2 O 5-Na 2 O porous nano-composites is examined. XRD and FT-IR spectroscopy indicated the successful formation of Al 2 O 3-P 2 O 5-Na 2 O crystallizes, while the incorporation of NiO/SiO 2 increased the total crystalline degree. TEM-results designate an increase in particle size of the phosphate-based crystallites as the NiO/SiO 2 level increases, which corresponded to the higher crystallization in the sodium aluminophosphate (Al 4 Na 7 P 9 O 32) nanocomposite. The FT-IR results showed that these modified sol-gel reactions are a result of acid hydrolysis of the Al-O-P and Na-O-P bonds and the influence of Si-O-OH, leading to network termination by POH groups and octahedrally solvated Al atoms (with strong quadrupole interaction) with the SiOH groups. With the introduction of NiO/SiO 2 , the fundamental band gaps are found to be 3.89, 3.93, 3.98, and 4.05 eV for direct allowed transitions. In the case of the indirect transition, the energy of the bandgap were 3.42, 3.57, 3.63, and 4.75 with the introducing of NiO/SiO 2. The saturation magnetization increased from 0.11 to 0.38 with increasing NiO content from 0.15 to 0.25%. Also, the switching field distribution (SFD) was investigated. The obtained optical and magnetic properties for Al 2 O 3-P 2 O 5-Na 2 O/NiO magnetic porous nanocomposites supported with silica which make them an excellent candidate for high-performance optics, biocompatible systems, solid-state electrolytes, fast ion-conducting materials , and opto-magnetic devices.
The expanding prevalence of heating, ventilation, and air conditioning (HVAC) systems, currently reliant on fossil fuel‐derived energy, necessitates energy‐saving measures to address climate change concerns. Implementing desiccants with high water vapor affinity reduces latent heat input, impacting the heat exchanger's functionality based on system and desiccant characteristics. This review analyzes the materials in solid desiccants, binders, and substrates, emphasizing advancements for enhanced heat transfer efficiency. Some notable materials explored include metal–organic frameworks, activated carbon, silica gel, polymers, and zeolite. In system level, the article discusses about the dual‐channel heat exchanger which emerges as a solution to drawbacks in existing dehumidification systems, offering higher performance and lower latent heat load in modern solid desiccant‐coated heat exchangers. Despite the environmental impact of material usage, future research should focus on sustainable materials, such as those derived from biomass or recycled sources, to maintain efficiency while minimizing waste in HVAC systems.
The introduction of fresh air into the indoor space leads to a significant increase in cooling or heating loads. Solid desiccant heat pump fresh air unit which can handle the latent and sensible load of fresh air efficiently have been proposed recently. To improve the performance of the solid desiccant heat pump fresh air unit in the fresh air handling process, in this paper, the application of composite silica gel in a heat pump fresh air unit was investigated. The comparison between silica gel coating (SGC) and composite silica gel coating (CSGC) shows that the adsorption rate and water uptake capacity of CSGC are more than two times higher than those of SGC. An experimental setup for the solid desiccant heat pump fresh air unit was established. The performance of SGC and CSGC was tested in the setup successively. Results show that under summer conditions, compared with the solid desiccant heat pump fresh air unit using silica gel (SGFU), the average moisture removal and COP of the one using composite silica gel (CSGFU) increased by 15% and 30%, respectively. Under winter conditions, compared with SGFU, the average humidification and COP of CSGFU increased by 42% and 17%. With optimal operation conditions of 3 min switchover time and 40 r/s compressor frequency, the COP of CSGFU under summer conditions can reach 7.6. Results also show that the CSGFU and SGFU have higher COP and dehumidification rate under higher outdoor temperature and humidity ratio.
Large single ANA crystals with perfect icositetrahedral morphology have been synthesized under a simple system without templates and hydrofluoric acid.
This minireview summarized the recent progress of converting the typical classes of large-scale aluminosilicates into zeolite materials for environmental remediation. The representative zeolite structures that can be directly converted from large-scale aluminosilicates via green chemistry approaches are addressed. The environmental remediation mechanisms on employing these zeolites for environmental remediation have been recapped. The present research gap and future research perspectives under carbon-neutral pressure via green chemistry principle on this topic are also discussed.
Zeolites are an emerging class of crystalline, nanoporous, inorganic material with regular micropores and highly tunable functionalities. Zeolites have been proposed as ideal materials for applications in catalysis, adsorption, and biomedicine due to their notable intrinsic features such as accessible void space, large surface area, and strong acidity. Microwave radiation is a robust technology that can quickly and efficiently prepare uniform and controllable zeolites that enable remarkable functions. We first review the current research progress in, and the fabrication of, zeolites using microwave technologies, and the related application. After a brief introduction to microwave theory and modification of zeolites, the discussion focuses on the five main microwave technologies for the synthesis of zeolites, that include microwave irradiation, microwave assisted hydrothermal synthesis, microwave assisted aging synthesis, microwave assisted ionothermal synthesis, and other microwave related methods. Some critical applications such as catalysis (NOx reduction, CO2 reduction, and Fenton), gas sensing and CO2 capture, and biomedicine are selected and discussed in detail. Finally, a summary and perspectives on research that may lead to the future development of this blossoming field.
We report effects of crystallization conditions and gel composition on the hydrothermal synthesis of large zinc-doped AlPO4-15 (ZnAlPO4-15) single crystals by using melamine (MA) as structure directing agent (SDA). By optimizing the following synthesis conditions: amount of MA, molar ratio of ZnO/Al2O3, molar ratio of P2O5/Al2O3, crystallization temperature and duration, the pure phase of ZnAlPO4-15 (size up to 95 μm) with good optical transparency can be obtained at 200 °C with 40 h in the system of Al2O3:1.07MA:0.27ZnO: 1.15P2O5:667H2O. To the best of our knowledge, it is the first report of synthesizing ZnAlPO4-15 by using MA as SDA.
Sustainable development of clean fuels demands a less energy-consuming procedure for the rapid synthesis of high-efficiency catalysts applied in the aromatization of olefins. Here, nanosized ZSM-5 aluminosilicates and GaZSM-5 gallosilicates catalysts were prepared via a seed-induced method by using conventional electric (CE) heating and microwave radiation (MW) heating. The effects of the heating strategy on the morphology, textural property and acidity of the synthesized samples, and also their catalytic performance of 1-hexene aromatization were studied. The results showed that the synergy of Brønsted acid site (BAS) and Lewis acid sites (LAS) provided by active GaO⁺ species in the catalysts played an important role in 1-hexene aromatization. MW heating can effectively promote the formation of well-dispersed GaO⁺ species located in the zeolite channels in proximity to BAS, providing higher dehydrogenation activity. Due to the closer spatial proximity and improved synergic effect between BAS and active GaO⁺ species, the nanosized GaNZ5-MW/MW sample prepared by two-step MW heating in both seed preparation and crystallization exhibited the highest BTX selectivity and the lowest coke-burning activation energy.
This work deals with the preparation of MAPO-5 zeolite by a hydrothermal method. In order to study the adsorption properties of this zeolite several transition metals (Fe, Co, Ni, Mn and Zn) were incorporated by a direct route. The obtained zeolites were characterized by various physico-chemical techniques such as X-ray diffraction, Fourier transform infrared spectroscopy, adsorption/desorption of argon, thermogravimetric analysis/differential thermal analysis, scanning electronic microscopy (FEG-ESEM) and Inductively coupled plasma–optical Emission spectrometry (ICP-OES). The prepared zeolites were tested for adsorption of water and xylene isomers (ortho-, para- and meta-xylene). The obtained results showed that the isomorphic substitution of the transition metals in the framework of the zeolite has been achieved with different levels of metals. The isomorphic substitution by zinc showed the best results in terms of the percentage of the incorporated metal, while the substitution by cobalt presented the lowest rate of incorporation in the framework of the zeolite CoAPO-5. The adsorption of water in the vapor state on the zeolite substituted by Ni, Fe and Zn has presented the best adsorption capacity. The adsorption of xylene isomers on MAPO-5 showed different isotherms, confirming that the adsorption behavior differs from one zeolite to another. ZnAPO-5 showed higher adsorption capacity for all xylene isomers with slight selectivity towards meta-xylene.
With a characteristic of S-shape isotherms for water vapor adsorption, aluminophosphate molecular sieve with AFI type (AlPO-5) has been considered as an up-and-coming adsorbent for the utilization in the adsorption cooling and heating (ACH) system. In order to improve its adsorption performance, iron doped AlPO-5 (FeAPO-5) with variable Fe loading was synthesized by microwave hydrothermal method. The effects of the iron doping content on the structure and adsorption performance were systematically investigated. The analysis results of X-ray diffraction (XRD), UV–visible spectroscopy (UV–Vis-DR) and scanning electron microscopy (SEM) showed that the iron ion was incorporated into aluminophosphate molecular sieves with a typical AFI structure as that of AlPO-5 through the isomorphous substitution. As the iron content of FeAlPO-5s increased, the isolated FeIII decreased, while oligonuclear FeIII and nanosized FeIII increased, besides, the static adsorption curves showed that the adsorption capacity of FeAlPO-5 increased first and then decrease. After iron modifying, the crystallinity and morphology of crystals was improved, and the adsorption performance increased from 0.185 g/g (AlPO-5A) to 0.235 g/g (FeAlPO-5), while the desorption performance of FeAlPO-5 (95.69 kJ/mol) was close to that of AlPO-5A (81.59 kJ/mol).
The kinetics of the thermal desorption of CO2 adsorbed on zeolite 13X were obtained using a differential thermogravimetric analyser under two different carrier gas conditions. The varying heating rates were set as 8, 12, 16, and 20 K min⁻¹, respectively. The desorption activation energy of the physisorption sites for this experiment evaluated by an integral method without prediction of the reaction order ranged from 12.15 to 14.12 kJ mol⁻¹ (CO2 as the carrier gas) and 43.32 to 50.42 kJ mol⁻¹ (Ar as the carrier gas), respectively. The desorption activation energy of the chemisorption sites ranged from 57.95 to 58.53 kJ mol⁻¹ (CO2 as the carrier gas) and 74.02 to 79.92 kJ mol⁻¹ (Ar as the carrier gas), respectively.
A review on solar powered solid desiccant – vapor compression hybrid cooling system has been carried out. This
provides latest information on recent development in air-conditioning and built environment produced by use of
hybrid cooling. Use of renewable solar energy can be a good source for regeneration heat provided in reactivating
desiccant dehumidifier used in alternate cooling systems. Desiccant dehumidification assisted vaporcompression
based hybrid air-conditioning systems can successfully couple to renewable solar thermal power to
dampen electricity use and to conserve environment in place of traditional air-conditioning. The energy saving
potential of solar powered desiccant cooling technologies is illustrated and discussed as compared to the vapor
compression based conventional cooling systems. It is found that the solid desiccant integrated hybrid cooling
systems are favourable than the conventional vapor compression cooling, because it is better means for economical and cleaner air-conditioning. The present review can ameliorate the usefulness of solar energy as renewable regeneration heat source in heat driven solid desiccant based hybrid cooling system and helps to guide future investigators to take the research opportunities in this area.
Chromium substituted nano-porous aluminophosphate (CrAPO-5) was investigated to discuss the effect of Cr³⁺ ions on the crystallization kinetics and crystal structure parameters by in situ time-resolved high resolution X-ray powder diffraction (HRXRPD). Results showed that the introduction of Cr³⁺ into pure AlPO4-5 influenced both the crystallization velocity and structural parameters of the resultant AFI. The presence of Cr³⁺ ions can severely retard the crystallization of AFI. The more Cr³⁺ ions in the gels, the stronger inhibitory effect would be. A kinetic analysis of the crystallization of CrAPO-5 was performed. A fair linear relationship between the activation energy and Cr³⁺ content was concluded. The calculated Avrami exponent n was in the range of 1–1.4, implying a similar 1D growth mechanism to that of AlPO4-5. The extracted structural parameters showed obvious crystal expansion, which might be caused by the trapping of Cr³⁺ ions in the framework. However, this influence on the structural parameters was limited. The a (=b) parameters increased to a maximum with chromium content, which is similar to the influence on the crystal growing velocity. The grain size variation of AFI differed from other metal substituted aluminophosphate molecular sieves, which implied a different state of chromium in the AlPO4-5 framework. The time-resolved HRXRPD proves to be a direct and efficient method to reveal the influence of Cr³⁺ ions on the crystallization and shows indirect evidence on the state of Cr³⁺ in the crystalized CrAPO-5.
With a characteristic of S-shaped water sorption isotherms, AlPO4-5 molecular sieves have been considered as up-and-coming adsorbents for the utilization in adsorptive cooling and heating systems. In order to avoid toxicity and corrosion of fluoride, this paper introduced a fluorine-free microwave hydrothermal synthesis strategy of pure AlPO4-5 crystals. The effects of hydrothermal conditions including template agent, crystallization temperature and time on the performances of AlPO4-5 adsorbents were systematically investigated. Fourier transform infrared spectroscopy, X-ray diffraction (XRD), ²⁷Al and ³¹P solid state magic angle spinning nuclear magnetic resonance (MAS NMR), pore size analyzer and Scanning electron microscopy (SEM) were used to determine the chemical structure, crystalloid phase, framework, pore structure and the morphology. Adsorption and desorption performances were measured by static adsorption, thermogravimetry and temperature programmed desorption. XRD results indicated that both triethylamine and tetraethylammonium hydroxide as templates, AlPO4-5 crystals could quickly be synthesized within 30–45 min under fluorine-free microwave irradiation. By comparison, the adsorption and desorption performance of the former (recorded as AlPO4-5A) was superior to that of the latter (recorded as AlPO4-5H). FT-IR, MAS NMR and SEM results revealed that AlPO4-5 crystals have the frameworks of alternating AlO4 and PO4 units and typical hexagonal rod-like morphologies.
In this paper, we consider the role of adsorbents in adsorptive cooling/heating (ACH) and analyse which are optimal for specific ACH cycles. The conclusions are formulated in terms of the optimum isobar shape, the Dubinin adsorption potential being used as a universal measure of the affinity between the adsorbent and the adsorptive. Selection/tailoring of the optimum adsorbents, creation of database of their properties as well as harmonization of the adsorbent and other components of ACH unit, cycle boundary conditions, and cycle management strategy are briefly discussed. New efficient adsorbents harmonized with particular ACH cycles are a prerequisite to achieving a major breakthrough in ACH, and therefore, are pivotal for this challenging application.
Sorption technologies, which are considered mainly for solar cooling and heat pumping before, have gained a lot of interests for heat storage of solar energy in recent years, due to their high energy densities and long-term preservation ability for thermal energy. The aim of this review is to provide an insight into the basic knowledge and the current state of the art of research on sorption thermal storage technologies. The first section is concerned with the terminology and classification for sorption processes to give a clear scope of discussion in this paper. Sorption thermal storage is suggested to cover four technologies: liquid absorption, solid adsorption, chemical reaction and composite materials. Then the storage mechanisms and descriptions of basic closed and open cycles are given. The progress of sorption materials, cycles, and systems are also reviewed. Besides the well-known sorbents like silica gels and zeolites, some new materials, including aluminophosphates (AlPOs), silico-aluminophosphates (SAPOs) and metal-organic frameworks (MOFs), are proposed for heat storage. As energy density is a key criterion, emphais is given to the comparison of storage densities and charging tempertures for different materials. Ongoing research and development studies show that the challenges of the technology focus on the aspects of different types of sorption materials, the configurations of absorption cycles and advanced adsorption reactors. Booming progress illustrates that sorption thermal storage is a realistic and sustainable option for storing solar energy, especially for long-term applications. To bring the sorption storage solution into market, more intensive studies in fields of evaluation of advanced materials and development of efficient and compact prototypes are still required.
Booming progress in the materials science offers a huge choice of novel porous solids which may be used for adsorption transformation of low temperature heat. This communication gives an overview of original and literature data on several classes of materials potentially promising for this important application, namely, metalaluminophosphates (AlPOs, SAPOs, MeAPOs), metal-organic frameworks (MIL, ISE, etc.), ordered porous solids (MCM, SBA, etc.), porous carbons and various composites (SWSs, AlPO-Al foil). For the SWS composites, we briefly considered the recent trends in their developing, namely, usage of host matrices with uniform pore dimensions and binary salt systems. We hope that this review will give new impulses to target-oriented research on the novel adsorbents for AHT and may also be beneficial for further consolidating international activities in materials science and heat transformation applications.
Aluminophosphates molecular sieves AlPO4-5 with novel morphologies of brooms and nano-fibers (3.8 µm and 70–200 nm in width, respectively) were hydrothermally synthesized by varying the precursor compositions, and used as templates to prepare hexagonal carbon micro and submicro tubes.
The brief review of innovative materials for adsorptive pumping and storage of low temperature heat as well as current trends in this field showed their great potential. To implement it in the near future it is reasonable to follow two complementary lines: (a) systematic testing for this application novel adsorbents initially developed for other targets (gas drying, separation, etc.), and (b) tailoring of new specific porous materials adapted just to variety of adsorptive cooling, heating and storage cycles under different climatic and boundary conditions. The main idea of the latter approach is that for each particular adsorptive cycle there is an optimal adsorbent, the thermodynamic characteristics of which could allow perfect performance of this cycle. The first step of the analysis is the formulation of requirements to this desirable (ideal) adsorbent. Then, the final step is to design and synthesize a new adsorbent with sorption properties close or even equal to those determined before as perfectly fitting the cycle. Particular examples of nanotailoring of adsorption properties are given for two phase materials, composites a salt in a porous host matrix", which can be used as efficient adsorbents of water, methanol and ammonia.
Rapid urbanization and higher thermal comfort requirements have sharply increased buildings’ energy demands. In tropical countries, for achieving energy efficient air-conditioning, desiccant dehumidifiers, which decouple sensible and latent load, are essential. While fixed-bed and rotary wheel desiccant dehumidifiers suffer from reduced adsorption capacity and lowered heat transfer efficiencies, desiccant coated heat exchanger (DCHE) system is a promising technology that can improve the energy efficiency of the standard vapor-compression air conditioners. Moreover, DCHEs also have prospective benefits in heat pump, adsorption chiller, and atmospheric water harvesting applications.
In this paper, the advantages of DCHEs over other dehumidifiers are established, and a comprehensive review of various desiccant materials and binders used in DCHEs is carried out. The possibilities of using advanced materials to achieve superior energy efficiencies are also underlined. Different methods of coating the desiccant on heat exchangers are described, and quantitative parameters required to analyze the performance of the system are presented. Operating parameters affecting the dehumidification and thermal performance are identified, and the effect of their variation on the dynamic behavior of DCHEs is elaborately studied. The significance of using mathematical models as design tools is recognized, and the underlying assumptions, applicability, and limitations of the models for simulating the complex heat and mass transfer phenomenon are highlighted. The possibility of integrating an additional heat transfer equipment such as sensible heat exchangers, heat recovery devices, solar evacuated tube collectors, cooling towers, and evaporative coolers with DCHE is also presented. Finally, the potential system-level applications of DCHEs as heat pumps, adsorption chillers, water harvesters, and standalone air-conditioners are assessed.
This review paper provides an update on recent developments in desiccant dehumidification and is intended to be a “one-stop” archive of known practical solutions that DCHEs can offer.
Desiccant wheel dehumidification air conditioning system makes full use of the principle of desiccant dehumidification and evaporative cooling. Besides desiccant material, the substrate also plays an important role in improving the dehumidification performance of a rotary desiccant wheel. The objective of this paper is to present the state of the art of substrate, and then provide guidelines for the choice and further research work of substrate. It is found that the substrate of a rotary desiccant wheel usually is porous ceramic fiber paper or glass fiber paper. To improve the dehumidification capacity of rotary desiccant wheel dehumidification system, substrate with high porosity and high thermal conductivity should be selected. Taking into account both dehumidification capacity and pressure drop, sinusoidal air channel is a good choice for rotary desiccant wheel. The review of non-rotary solid desiccant dehumidification system reveals that substrate plays an important role in dehumidification processing and porous fiber paper is the best choice for substrate of rotary desiccant wheel. Short service life and poor adsorption performance are the main drawbacks of existing desiccant wheels. The match for substrates and desiccant materials has not been understood deeply. In the future, revealing the nature of the effect of substrate on the dehumidification performance may be the main work to fill in these blanks.
We studied the water-adsorption properties of crystalline aluminophosphates (in brief, "AlPO4") containing ordered micropores, focusing on the effects of porous structure and heteroatoms incorporated in AlPO4 frameworks. For this investigation, AlPO4 with AFI- and CHA-type structures were hydrothermally synthesized. Also, the AFI-AlPO4 had Si, Fe, Mg, Co, or Cr incorporated at controlled amounts (0-5 mol%). Water adsorption isotherms of the AlPO4 samples were taken at 308 K, showing S-shape curve which is typical characterization of nanomaterial containing uniform nanopores. The amount of water adsorbed in AlPO4 was dependent on the specific surface area. The CHA-AlPO4 exhibited much higher water adsorption capacity than the AFI-AlPO4. The inflection-point humidity of the S-curve was also dependent on the microporous structure. In the case of AFI-AlPO4, the inflection point differed markedly depending on the amount and type of the heteroatoms incorporated in the frameworks. In a group with same micropore structure, the S-curve inflection point decreased in the order of "solely aluminophosphate"> "Co-doped" ≈ "Crdoped" > "Fe-doped" > "Mg-doped" > "Si-doped." The decrease in the inflection point humidity was found to be positively correlated to the amount of the heteroatom incorporated. Different inflection point humidities might be attributed to changes in the affinity between the AlPO4 framework and water molecules as a result of incorporation of heteroatoms. The results of this study show that it is possible to control the water adsorption properties of AlPO4 precisely, which would be very helpful in developing suitable water adsorbents for use in water adsorption chilling systems under specific working conditions.
Fixed bed and rotatory desiccant systems have been widely studied and used for dehumidification; they suffer from decreasing sorption capacity as the desiccant's temperature increases due to the released heat of adsorption. The desiccant coated heat exchangers (DCHX) overcome this limitation. Such heat exchangers are able to deliver combined heat and mass transfer between the process air and the working fluid. The process air can be cooled and dehumidified simultaneously by pumping cooling water/refrigerant in the DCHX. The DCHX has to be heated cyclically to regenerate the desiccant material.
This paper presents a review on the studies conducted on air-to-liquid DCHXs. It summarizes various modeling approaches used to simulate the performance of DCHX as well as the experimental studies conducted to validate these models. It also reviews the current and potential applications of these heat exchangers. Current work in this field consists of experiments conducted on the DCHX as standalone equipment (i.e. component level) as well as an integrated component into cooling and dehumidification systems (system level). The integration of the DCHX in such systems was found to improve the COP, leading to energy savings.
Porous aluminophosphates (AlPOs) and silicoaluminophosphates (SAPOs) with zeolite-like structures have received considerable attention as potential adsorbents for heat transformation applications using water adsorption/desorption cycles. Since a detailed experimental characterisation of the water adsorption properties has only been performed for some of these materials, such as AlPO-18 (AEI topology) and SAPO-34 (CHA topology), more systematic insights regarding the influence of the pore topology and (for SAPOs) the arrangement of the framework protons on the affinity towards water are lacking. To study the relationships between structure and properties in more detail, the interaction of water with six structurally different AlPOs (with AEI, AFX, CHA, ERI, GIS, RHO topologies) and their SAPO analogues was investigated using dispersion-corrected density-functional theory (DFT-D) calculations. Different possible locations of silicon atoms and charge-balancing protons were considered for the SAPO systems. The calculations for SAPOs at low water loadings (one H2O molecule per framework proton) revealed that the interaction energies exhibit a considerable variation, ranging from -75 to -100 kJ mol(-1) (per water molecule). The differences in interaction energy were rationalised with the different structural environment of the framework protons at which the water molecules are adsorbed. At high water uptakes (near saturation), interaction energies in the range of -65 kJ mol(-1) were obtained for all AlPOs, and there was no evidence for a marked influence of pore size and/or topology on the interaction strength. The interaction of water with SAPOs was found to be approximately 5 kJ mol(-1) stronger than for AlPOs due to an increased contribution of electrostatic interactions. An analysis of the structural changes upon water adsorption revealed striking differences between the distinct topologies, with the materials with GIS and RHO topologies being distorted much more drastically than the systems based on double six-ring (d6r) units. Moreover, the direct coordination of water molecules to framework aluminium atoms occurs more frequently in these materials, an observation that points towards a reduced structural stability upon hydration.
Desiccant dehumidification system is an alternate option against conventional dehumidification system in hot and humid climates. Conventional dehumidification systems have many drawbacks that include high power consumption and increase the chlorofluorocarbon (CFC) level in the environment and major contribute to depletion of ozone layer. This paper discuss the functioning of dehumidification, cooling and air-conditioning systems using various solid desiccant with focus on the use of solar energy for dehumidification of humid air and regeneration of solid desiccant wheel. A comparative study of various dehumidification, cooling and air-conditioning systems show that solid desiccant has low operating and maintenance cost and is environment friendly.
In this paper, a desiccant coating with lithium and magnesium modified ion exchange resin (MIER) on finned tube heat exchanger was prepared by impregnation method and electrostatic spraying process, respectively. Impregnation condition and spraying process, which would badly affect the adsorption performance of MEIR desiccant coating, were investigated in detail. Also the morphology, structure, composition and thermal performance of desiccant coating were characterized by X-ray diffractometer (XRD), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS) and temperature programmed desorption (TPD), respectively. Besides, its cycle stability was also taken into consideration. The results manifested that electrostatic spraying technique and impregnation method can be employed to prepare a thick and uniform desiccant coating (120μm) with modified ion exchange resin on finned tube heat exchanger successfully, which had an excellent cycle stability. The results of TPD analysis showed, for the enhanced interaction between the modified ions and water molecules, desorption activation energy of the modified resin can be raised slightly. And the static and dynamic adsorption curves of modified desiccant coating indicated that, whether in high or low humidity environment, the adsorption performance could be significantly improved after modification.
A DCHE (desiccant coated heat exchanger) is a novel solid desiccant component with desiccant coated onto the surface of a fin-tube heat exchanger, in which desiccant coated aluminum-fin is an important part. It has an impact on overall performance of the whole desiccant cooling system. In this study, different zeolite-like powders (SAPO-34 and FAPO-34) coated aluminum sheets were fabricated. Their performance was tested and analyzed to investigate the feasibility in a DCHE system. Experimental values of surface area and pore parameters for desiccant coatings had a slight reduction because of the existence of binder solution. Adsorption kinetics indicated that the binder solution would not affect water adsorption capacity or adsorption kinetics of original desiccant powder. Moreover, SAPO-34 and FAPO-34 coated aluminum sheets had higher adsorption capacity than that of silica gel. Sorption isotherms were also measured and fitting equations were developed based on Polanyi principle. Finally, their dehumidification performance in DCHE systems was predicted with a mathematical model. Simulation results indicated that the DCHE prepared from FAPO-34 can have 2–3 times larger dehumidification capacity than those from SAPO-34 and silica gel at low regeneration temperatures.
This work reports the results of an experiment study on anti-corrosion coatings obtained by the direct growth of zeolite Y films on aluminium substrates by an amine-modified hydrothermal synthesis. In particular, the effect of the TEA (triethanolamine) presence was evaluated in terms of coating performances by varying synthesis times and drying temperatures. The morphological analysis showed that the films were homogeneous and compact up to 120 μm in thickness. Peel tests confirmed the good adhesive properties of all kinds of investigated coatings. In the end, electrochemical tests, carried out in 3.5% NaCl and in Ca(OH)2 saturated solution, showed good barrier properties offered by this type of coating. Low drying temperature allowed to keep TEA entrapped in the zeolite structure so that it greatly influenced corrosion protection behaviour.
AlPO4-5 zeolite was successfully employed in a resistance type humidity sensor. AlPO4-5 zeolite humidity sensor was fabricated by coating nanosized materials over interdigitated electrodes. The sensor shows good linearity over a relative humidity (RH) range from 11% to 95%. At the frequency of 100 Hz, response and recovery time are both about 2 s, and the maximum hysteresis is <3%. In nanoscale confinement range, the sorbed water quantity in AlPO4-5 channels is exceptionally large and the water sorption phenomenon looks like “capillary condensation” phase transition. The complex impedance at different RH was investigated to understand the sensing mechanism. The results indicate the potential application of AlPO4-5 zeolite for fabricating high-performance humidity sensors.
The aluminophosphate type zeolitic material AlPO4-5 was synthesized by hydrothermal method using aluminum dross as a raw material and its water vapor adsorption property was studied. Porous AlPO4-5 and nonporous AlPO4 were formed and the selectivity of their formation was determined by varying the reactant ratios. Both AlPO4-5 and tridymite type AlPO4 (nonporous phosphate) were obtained from aluminum dross when the reaction products were synthesized with the following composition Al2O3: P2O5: Triethylamine (TEA): H2O = 1:1:1:40. While AlPO4-5 was preferentially synthesized as a main product when the compositions were Al2O3: P 2O5: TEA: H2O = 2:1:1:40 and 1:1:2:40. The variation in the reactant ratio dictated the specific surface area of the resulting reaction products. The formation of AlPO4-5 with a composition of 1 : 1 : 2 : 40 generated surface area of 353 m2/g. The reaction products synthesized at 2 : 1 : 1 : 40 and 1 : 1 : 1 : 40 generated surface areas of 187 and 75m2/g respectively. All the aluminophosphate products exhibited the type IV or V isotherm by IUPAC classification for water vapor adsorption due to the AlPO4-5 pore with strong affinity. This study demonstrated the efficacy of AlPO4-5 obtained from aluminum dross as desiccant that is the aluminum dross-derived AlPO4-5 demonstrated high adsorption capacity and adsorption properties suitable as an adsorbent or a desiccant.
AlPO-5 molecular sieves were prepared by the hydrothermal reaction of a gel mixture with the following compositions: Al2O3 : P2O5 : Et3N : H2O = 1:1:1.5:x, where x is between 100 and 750 H2O molar ratio. The structure and morphology of the AlPO-5 molecular sieves depend on the gel mixture's composition, hydrothermal temperature, hydrothermal reaction time, and pH. Without pH control, the AlPO-5 structure changed from a spherical shape at H2O = 100 to a hexagonal pillar shape at H2O = 450. With pH control in the range of about 2.5-3.5, the hexagonal pillar crystals began to form at H2O = 100 and an island of hexagonal pillars with radiation form appeared at H2O = 300-450 due to the formation of a tridymite type of dense AlPO4 phase. It appears that the formation rate of hexagonal pillar crystals to form a dense AlPO4 phase is favorable under acidic conditions, and an amorphous AlPO-5 structure forms under basic conditions. Thus, the H2O concentration and pH value have a dramatic effect on the AlPO-5 structure.
A summarizing overview is presented for our investigations of water adsorption on aluminophosphate molecular sieves of AFI structure type. The mechanism of water adsorption is mainly controlled by the amount and not the kind of the substituted heterometal. The initial stage of the process consists of a layer-like adsorption. This is followed by (i) a phase transition to liquid-like water at a definite relative pressure dependent on a substitution level and (ii) final volume filling of micropores. Adsorbed water forms islands around the metal centers, especially under low relative pressures. This leads to an apparent effect of averaging the adsorption between the layer-like one and the volume filling of micropores, being more pronounced with higher metal contents of the crystals. The effect is reflected in a gradual transition of the isotherms from type IV or V to type I. Neutral molecular sieves with Me3+ centers substituted for Al3+, e.g. Cr3+ or Fe3+, exhibit no significant differences in adsorption of water as compared to the charged materials with Me2+ or Me4+ centers substituted into the framework. The regular changes of adsorption isotherms of water, following variation in the heterometal content, can be used as evidence for framework incorporation of the heterometals. © 2005 Academie des sciences. Published by Elsevier SAS. All rights reserved.
Hydrothermal synthesis of pure AlPO4-5 crystals using various reaction gels is reported. The influence of synthesis conditions on the purity of the AlPO4-5 has been studied by changing Al sources and factors affecting the pH of the starting gel. It was observed that the additive acid to adjust the gel pH value has strongly influenced both the crystallinity and purity of the resulting AlPO4-5 crystals. The crystals were characterized by XRD, FT-IR, TGA, BET, SEM and TPD techniques. The resulting microporous AlPO4-5 was used as an efficient support for a functional polymer to produce an efficient heterogeneous basic nanocomposite catalyst. The catalytic activity of this novel nanocomposite was tested for Knoevenagel reaction under solvent-free conditions at room temperature. The catalyst showed a considerable degree of reusability besides very good activity.
In order to increase the efficiency of adsorption heat pumps, an arrangement involving zeolite 4A synthesized on metal heat exchanger tubes was recently proposed and a related mathematical model was presented. In this study, the same mathematical model was employed to determine the effects of the metal type and the thermal and mass diffusivities of the adsorbent on the performance of the adsorption heat pumps. The calculations were performed for two different wall thickness values of the heat exchanger tubes and various zeolite layer thicknesses. Copper, nickel and stainless steel heat exchanger tubes were tested for their suitability to be used in the system, and the effects of the thermal and mass diffusivities related to the adsorbent were investigated in the ranges of 0.0004 to 0.0023 cm2 s−1 and 4×10−7 to 1×10−5 cm2 s−1, respectively. As a result, the diffusion coefficient was observed to be the limiting factor for the adsorption heat pumps employing the zeolite–water pair in determining the optimum thickness of the zeolite layer synthesized on metal heat exchanger tubes, while the effects of the metal type and the thermal diffusivity of the adsorbent were observed to be of trivial importance.
The utilization of the reversible chemical and physical sorption of water on solids provides a new thermal energy storage concept with a great potential for lossless long-term storage. The performance of microporous aluminophosphates in heat storage applications is highlighted by a comparative thermogravimetric and calorimetric study of three known materials (SAPO-34, AlPO4-18, APO-Tric) and is correlated with their structural features. The maximum water sorption capacity is similar for all three samples and results in a stored energy density of 240 kWh m-3 in the 40140 degrees C range. The elemental composition influences the gradual (silicoaluminophosphate SAPO-34) or sudden (aluminophosphates AlPO4-18, APO-Tric) water uptake, with the latter being favourable in storage systems. The driving force for the determined sorption process is the formation of highly ordered water clusters in the pores, which is enabled by rapid and reversible changes in the Al coordination and optimal pore diameters. The ease with which changes in the Al coordination can occur in APO-Tric is related to the use of the fluoride route in the synthesis. The understanding of these fundamental structure/sorption relationships forms an excellent basis for predicting the storage potential of numerous known or new microporous aluminophosphates and other porous materials from their crystal structures.
The synthesis of the chromium-substituted aluminophosphate [Cr]APO-5 (AFI) using microwave irradiation was investigated systematically. The prepared materials were characterized by chemical analysis (ICP-AES), XRD, SEM, DR–UV–Vis spectroscopy, and sorption of different probe molecules, i.e., water, benzene, and nitrogen. Stable chromium incorporation into the AFI-framework can be achieved up to a chromium content of at least 3.2mol% (5.6wt.%). Besides the content of chromium and water, the nature and content of co-templates such as acetic acid or ethylene glycol in the gel for the microwave-assisted synthesis were varied. The concentration of the co-template is a key factor for successful [Cr]APO-5 synthesis. The microwave-assisted crystallization of [Cr]APO-5 was completed within 30min whereas the conventional hydrothermal crystallization took at least 6h for completion. Nevertheless, the yields and quality of [Cr]APO-5 obtained from hydrothermal and microwave-assisted synthesis are comparable.
We report the hydrothermal synthesis of AlPO4-5 crystals using TBAOH as template. The resulting product sensitively depends on the gel compositions and synthesis procedure. By treating the tri-isopropylate aluminum gel solution with ultrasonic, and adding HF acid to adjust the gel pH value, perfect hexagonal-shaped AlPO4-5 crystals are obtained. At high TBAOH content, some centrosymmetric intersecting crystals are observed in the products, this can be interpreted by the polar growth of AlPO4-5 crystals. The decomposition of template TBAOH in the channels of AlPO4-5 crystals has been studied by FTIR and TG analysis.
A novel method is proposed for the preparation of zeolite coatings on substrates, which suppresses the reaction in the bulk and promotes that on the substrate by applying a temperature difference between the reaction mixture and the substrate. The substrate is heated directly, with a soldering resistance in this study, while the reaction mixture is kept at a lower temperature by means of a water bath. The method is tested for the case of zeolite 4A synthesis on stainless steel plates from a clear aluminosilicate solution. As a result of suppressed activity, the composition of the solution remains nearly constant and the solution side of the coating always experiences a lower temperature. As a result, the phase transformations of the metastable zeolites can be delayed for extremely long periods of time, depending on the volume of the reaction mixture. The deposition of crystals from the solution onto the surface of the coating is also avoided. Crystalline coatings of zeolite 4A could be prepared by the appropriate selection of the temperatures of the water bath and the substrate. A final treatment, which involves an increase in the temperature of the water bath, may be applied to the coatings at the end of their preparation, in order to remove the sparse amount of impurities observed at certain synthesis times and to increase the crystallinities when necessary. The mass of the zeolite coated on the substrate is shown to increase significantly with respect to that obtained by conventional synthesis when the proposed method is utilized. The method can thus be useful for some applications where the employment of relatively thicker zeolite coatings of metastable phases may be desired.
An arrangement involving zeolite synthesized on metal wire gauzes was recently proposed to eliminate the limitations caused by insufficient heat transfer within the solar collector and a related mathematical model was presented. According to such an arrangement, zeolite 4A is envisioned to be synthesized as continuous coatings on stainless steel wire gauzes which are assumed to be placed in the collector vertically in a manner assuring a firm contact between the absorber plate and the wire gauzes. In this study, the same mathematical model is employed to determine the efficiency of the solar adsorption heat pumps under various conditions and the results are compared with those obtained by assuming the employment of a conventional arrangement leading to various amounts of thermal gradients in the solar collector. The effects of the thermal gradients on the performance of the system are investigated by taking into account the employment of zeolite 4A and zeolite 13X, and by varying the weather conditions as well as the difference between the condenser and evaporator temperatures. As a result, the presence of the thermal gradients in a solar collector is observed to decrease the COPsolar values, indicators of the efficiency of the system, to a great extent, especially when a large amount of zeolite is to be utilized. In the case of the employment of zeolite 13X, the adverse effects of thermal gradients on COPsolar are more pronounced.
Zeolites and mesoporous materials were systematically modified by ion exchange and impregnation with hygroscopic salts to improve their storage capacity in thermochemical storage of heat. The sorption properties of those potential storage materials against water were investigated with physico-chemical methods such as thermogravimetry, differential scanning calorimetry, microcalorimetry and isotherm measurements confirming the improvement of the storage properties. Selected materials were pelleted and investigated in a lab-scaled storage to prove the performance of the modified storage materials. Two zeolite based materials with a charging temperature of approximately 450 K but a high temperature lift and an improved storage density of up to 576 kJ/L were found. Impregnated mesoporous materials show a much lower temperature lift, however, they offer a potential storage density of about 864 kJ/L at a charging temperature of about 390 K.
As a promising, environmentally friendly alternative for the toxic and carcinogenic chromate conversion surface finishing, zeolite coatings on various metal alloys have shown excellent corrosion resistance, strong adhesion to substrates, and extraordinary thermal and mechanical properties. This review summarizes recent technological advances of zeolite corrosion-resistant coatings from the conventional hydrothermal in situ synthesis method to the newly developed ambient pressure deposition process. The topic of zeolites as smart inhibitor pigments is also discussed. Future perspectives of zeolite corrosion-resistant coatings are envisaged at the end of this review.
Additional component(s) introduced into the reaction gel allow for a stable framework substitution of Cr in significant amounts. Appropriate component(s) and metal compounds, especially those of Al, are primary factors determining the substitution. The products of the system {triethylamine−acetate ions−Cr3+ ions} have been characterized in more detail and compared with those from other synthesis procedures. The Cr heteroatoms are 4-fold bonded to the framework and their coordination is complemented by two ligands from the pores (water molecules in the calcined form) to give the favored octahedral-like coordination. The coordination can be changed reproducibly from 6 to 4 by heating the crystals over 550 K (dehydration) and vice versa by cooling (hydration). The framework Cr(III) cannot be oxidized and causes no framework charge. The reaction conditions, differing from those in the classical syntheses, enable the control of the crystal morphology. The crystals can be grown as flat hexagonal pellets of a perfect morphology with high crystallinity and sorption properties.
The microwave-assisted hydrothermal synthesis of Cu-containing AFI structures AlPO4-5 and SAPO-5 in good product quality was successfully accomplished. The modification of texture, acidity and redox properties caused by incorporation of Cu was examined using XRD, SEM, TG/DTA, N2 adsorption, UV–vis- and FTIR spectroscopy. Copper impregnated AlPO4-5 and SAPO-5 served as reference samples. Pyridine and CO were used as probe molecules to characterize acidity and nature of Cu species. XRD patterns of the calcined, template-free samples confirmed the crystallinity and the absence of any crystalline copper phase (Cu, Cu2O, CuO). Surface areas and pore volumes indicate the absence of pore blocking by amorphous material or agglomerated Cu species. Brønsted acid sites were created in CuAPO-5 confirming isomorphous substitution of Cu2+ into the lattice. Cu2+/Cu+ with Lewis acid character could be identified by FTIR spectroscopy using pyridine as probe molecule. Pre-reduction with CO and H2 at various temperatures allowed a differentiation of Cu sites. An essential part of the Cu is located on non-lattice positions with high dispersion in form of isolated Cu2+. Reduction with CO revealed the existence of Cu2+ sites bearing oxygen (e.g., Cu2+–O–Cu2+). Reduction with H2 and subsequent CO adsorption leads to Cu aquacarbonyls.
The water sorption isotherm over AlPO4-5, in which a low sorption capacity in the initial region followed by a steep rise in sorption is observed, is different from those obtained for polar molecules, namely acetonitrile, ammonia and methanol. An attempt has been made to explain the water sorption behaviour on the basis of a condensation phenomenon using the structural characteristics of AlPO4-5.
AlPO(4)-5 and its chromium-containing analogue, CrAPO-5, were prepared using aluminum dross in the presence of triethylamine as a structure directing agent; both Al dross powder and an extracted precipitate from Al dross in an alkali solution were used as an aluminum source. These materials were characterized by XRD, BET surface area measurement, SEM, EDX, XRF, ESR, and UV-vis spectroscopy. Their textural properties were found close to those of the corresponding reference samples prepared using pure Al(OH)(3). CrAPO-5 samples prepared from the aluminum dross were tested as a catalyst for liquid phase oxidation of tetralin using t-butyl hydroperoxide as an oxidizing agent, which showed virtually identical catalytic performances to that obtained by a reference CrAPO-5 catalyst.
The synthesis and microstructure manipulation of SAPO-5 films on porous alpha-Al2O3 substrates (samples F-1 through F-6) have been studied with the main focus on the effect of Si content, substrate setting, and the structure-directing function of a precoated chitosan thin layer. Without the chitosan layer, the supported SAPO-5 film synthesized under traditional heating is not continuous with either pin holes or crystal boundaries. The preferentially c-oriented SAPO-5 top-layer over a closely packed layer of SAPO-5 crystals was synthesized on the porous alpha-Al2O3 substrate using the chitosan thin layer as the structure-directing matrix. According to the characterization using scanning electron microscopy (SEM) and X-ray diffraction (XRD), the thickness of the c-oriented SAPO-5 top-layer is changed from 1.5 to 10 microm while keeping the thickness of the packed layer of SAPO-5 crystals almost unchanged. Electron probe microanalysis (EPMA) demonstrates that P, Al, Si, and C contents within the two-layer film are uniformly distributed. The percentage of c-oriented crystal grains in the resulting SAPO-5 films evaluated by using the pole-figure analysis is over 70% (71.2% for F-4, 74.2% for F-5, and 75.9% for F-6). On the basis of our experimental observations, it has been indicated that the gradual dissolution of chitosan in the acid precursor solution is accompanied with the formation of the SAPO-5 top-layer and the packed layer of SAPO-5 crystals in the hydrothermal reaction, and both the surface and the entire body of the chitosan layer serve as a three-dimensional structure-directing matrix.
The influence of both bivalent and trivalent metal substituents from a range of metal cation (Co, Mn, Mg, Fe and Cr) on the acidic property (both Brönsted and Lewis) of metal substituted aluminum phosphate MeAlPOs is monitored. The influence of the environment of the acid site is studied both by localized cluster and periodic calculations to propose that the acidity of AlPOs can be predictable with accuracy so that AlPO material with desired acidity can be designed. A semi-quantitative reactivity scale within the domain of hard soft acid-base (HSAB) principle is proposed in terms of the metal substitutions using density functional theory (DFT). It is observed that for the bivalent metal cations Lewis acidity linearly increases with ionic size, where as the Brönsted acidity is solely dependent on the nearest oxygen environment. Intramolecular and intermolecular interactions show that once active site of the interacting species is identified, the influence of the environment can be prescribed. Mg(II)-doped AlPO-34 shows highest Brönsted acidity and whereas Cr(III)-doped species shows lowest acidity. Fe(II)/Fe(III)-doped AlPO-34 shows highest Lewis acidity, whereas Mn(III), Mg (II) shows lowest acidity.