Desorption Enthalpy Research Articles

Hydrogen storage thermodynamics and kinetics of the as-cast and milled Ce-Mg-Ni-based alloy

Aiming at investigating the improvement of hydrogen storage performance of the ternary Ce5Mg85Ni10 alloy after mechanical ball milling, the phase transformation and microstructure evolution of the milled alloys are observed by XRD, SEM and TEM. After ball milling, the alloys mainly exhibit nanocrystalline and amorphous structures and consist of Mg, Mg2Ni and CeMg12 phases. After hydrogen absorption, the milled alloys consist of MgH2, Mg2NiH4 and CeH2.73 phases. Isothermal and non-isothermal reaction curves of hydrogen absorption and desorption of the milled alloys are characterized by an Sievert apparatus, DSC and TGA. The Arrhenius and Kissinger equations are used to calculate the hydrogen desorption activation energy. For the Ce5Mg85Ni10 alloy, its structure and thermodynamics and kinetics performance of hydrogen storage strongly depend on the ball milling time. The results shows that ball milling for 5 h can achieve the optimal activation and kinetics performances but further prolonging the ball milling time lead to the impairment of hydrogen storage performance. Besides, ball milling time has little effect on thermodynamic property. Experiments results show that the 5 h-milled alloy after activation can absorb 4 wt% H2 in 18 s at 473 K and 3 MPa, and can desorb 3 wt% H2 in 906 s at 533 K and 1 × 10−4 MPa. In addition, the hydrogen desorption enthalpy change of the 5 h-milled alloy is 75.28 kJ/mol and the hydrogen desorption apparent activation energy of this alloy is 62.69 kJ/mol.

Improved hydrogen storage thermodynamics and kinetics of La–Ce–Mg–Ni alloy by ball milling

Reducing particle size and modifying surface state by ball milling are recognized as a useful method to improve the hydrogen absorption kinetics of Mg-based alloys. In order to obtain amorphous and nanocrystalline structures, ball milling was used to process the as-cast La7Ce3Mg80Ni10 alloy ingot. The effects of ball milling time (5–30 h) on the structures and the thermodynamics and kinetics of hydrogen absorption/desorption of the as-milled alloys were studied. The XRD, SEM and TEM were applied to investigate the structure of the alloys. The Sievert apparatus, DSC and TGA were applied to study the isothermal and non-isothermal properties of hydrogen absorption/desorption. The Arrhenius and Kissinger methods were used to calculate the activation energy of hydrogen desorption. The results indicate that the 10 h-milled alloy has the optimal activation performance and hydrogen absorption/desorption kinetics. Prolonging or shortening the ball milling time results in the impaired hydrogen storage properties. The 10 h-milled alloy in fully activation state can absorb 4 wt% H2 in 80 s at 473 K and 3 MPa, and it can desorb 3 wt% H2 in 191 s at 573 K and 1 × 10−4 MPa. The hydrogen desorption enthalpy (ΔH) of the 10 h-milled alloy is 74.21 kJ/mol. Moreover, the 10 h-milled alloy has the smallest apparent activation energy for hydrogen desorption (62.9 kJ/mol), which is the reason why it has the optimal hydrogen storage properties.

Efficient SO2 capture at ultra-low concentration using a hybrid absorbent of deep eutectic solvent and ethylene glycol

Deep eutectic solvents (DESs) are considered as the highly effective absorbents for sulfur dioxide (SO2) capture. However, the high viscosity of DESs and the resulting slow absorption rate as well as low absorption capacity at low SO2 concentration seriously hinder their industrial application. In this study, DES of N-methyldiethanolamine (MDEA) and imidazole (Im) is simply blended with ethylene glycol (EG) forming a hybrid absorbent, namely MDEA/Im-EG, which exhibits extremely high SO2 capture capacity at low concentration. In particular, SO2 capture capacity in MDEA/Im-EG (molar ratio = 1:1) reaches 0.446 g SO2/g absorbent at 293.2 K with SO2 concentration of 2000 ppm. Moreover, the corresponding desorption enthalpy is only −40.67 kJ/mol. To well understand the results, thermodynamic analysis of SO2 capture is performed and the SO2 capture mechanism is speculated by nuclear magnetic resonance and Fourier transform infrared spectroscopy.

Enhanced hydrogen storage properties of Ti–Cr–Nb alloys by melt-spin and Mo-doping

Ti–Cr–Nb hydrogen storage alloys with a body centered cubic (BCC) structure have been successfully prepared by melt-spin and Mo-doping. The crystalline structure, solidification microstructural evolution, and hydrogen storage properties of the corresponding alloys were characterized in details. The results showed that the hydrogen storage capacity of Ti–Cr–Nb ingot alloys increased from 2.2 wt% up to around 3.5 wt% under the treatment of melt-spin and Mo-doping. It is ascribed that the single BCC phase of Ti–Cr–Nb alloys was stabilized after melt-spin and Mo-doping, which has a higher theoretical hydrogen storage site than the Laves phase. Furthermore, the melt-spin alloy after Mo doping can further effectively increase the de-/absorption plateau pressure. The hydrogen desorption enthalpy change ΔH of the melt-spin alloy decreased from 48.94 kJ/mol to 43.93 kJ/mol after Mo-doping. The short terms cycling test also manifests that Mo-doping was effective in improving the cycle durability of the Ti–Cr–Nb alloys. And the BCC phase of the Ti–Cr–Nb alloys could form body centered tetragonal (BCT) or face center cubic (FCC) hydride phase after hydrogen absorption and transform to the original BCC phase after desorption process. This study might provide reference for developing reversible metal hydrides with favorable cost and acceptable hydrogen storage characteristics.

Study on the Calculation method of the Coal and Gas Outburst Threshold

The four undetermined parameters of the temperature-pressure-adsorption equation (TPAE) were obtained by regression of the series isothermal adsorption data of the measured middle-rank deformed coal and normal coal. The unit isosteric adsorption enthalpy (UIAE) was obtained. According to the negative value of the UIAE, adsorption is an exothermic process, desorption is an endothermic process. The coal and gas outburst threshold (OT) is defined as the product of the firmness coefficient and the unit isosteric desorption enthalpy (UIDE) to show the combined effect of adsorption thermodynamics and tectonic stress. The UIDE difference between the two coals was from 1.91 times expanded to OT difference of 5.74 times.

Enhanced hydrogen properties of MgH2 by Fe nanoparticles loaded hollow carbon spheres

In the present investigation, we have reported the synergistic effect of Fe nanoparticles and hollow carbon spheres composite on the hydrogen storage properties of MgH2. The onset desorption temperature for MgH2 catalyzed with hollow carbon spheres and Fe nanoparticle (MgH2-Fe-HCS) system has been observed to be 225.9 °C with a hydrogen storage capacity of 5.60 wt %. It could be able to absorb 5.60 wt % hydrogen within 55 s and desorb 5.50 wt % hydrogen within 12 min under 20 atm H2 pressure at 300 °C. The desorption activation energy of MgH2-Fe-HCS has been found to be 84.9 kJ/mol, whereas the desorption activation energies for as received MgH2, Hollow carbon sphere catalyzed MgH2 and Fe catalyzed MgH2 are found to be 130 kJ/mol, 103 kJ/mol, and 94.2 kJ/mol respectively. MgH2-Fe-HCS composite lowered the change in enthalpy of hydrogen desorption from MgH2 by 20.02 kJ/mol as compared to pristine MgH2. MgH2-Fe-HCS shows better cyclability up to 24 cycles of hydrogenation and dehydrogenation of MgH2. The mechanism for the better catalytic action of Fe and HCS on MgH2 has also been discussed.

Enhanced hydrogen storage performance of Li and Co functionalized h-GaN nanosheets: DFT study

Based on the density functional theory (DFT) computations, we investigated the hydrogen storage performances of alkali metal (Li) and transition metal (Co) decorated the defective GaN nanosheets. Fundamental aspects including the interaction properties, bonding characteristics, adsorption ability, frontier orbital, HOMO–LUMO energy gaps, natural bond orbital (NBO) analysis, projected densities of states (PDOS) and statistical thermodynamic stability have been demonstrated to analyze the interaction properties of H2 molecules. As a theoretical strategy, non-covalent interactions (NCI) and the atoms in molecules (QTAIM) descriptors were performed to depict weak interactions. The LiN-GaN and CoN-GaN systems and the H2 uptake capacity revealed to be 7.78% and 5.55%, respectively. Our results demonstrated that H2 molecules are introduced sequentially on the Li and Co that functionalized both sides of VN-GaN nanosheets yielded the gravimetric densities up to 8.158% (2Co-VN-GaN) that well above the gravimetric DOE achieve. The 2LiN-GaN and 2CoN-GaN are energetically more effective for the H2 adsorption, stable and preferred than pristine GaN nanosheet.Additionally, two binding mechanisms including polarization of the hydrogen molecules and σ orbitals hybridization of H2 molecules have been investigated to explain the interaction of H2 molecules. The hydrogen desorption enthalpy and desorption temperatures of hydrogen molecules, indicating the H2 molecules are easy to desorb from Li and Co decorated defective GaN nanosheets. These results suggest the possibility of an excellent and promising nanostructural material to improve the performance of hydrogen storage for in fuel cells application at ambient temperature.

Determination of the regeneration energy of direct air capture solvents/sorbents using calorimetric methods

Regeneration energy and recyclability are important parameters that dictate the viability of a CO₂ direct air capture (DAC) process. In this study, differential scanning calorimetry (DSC) is used in conjunction with thermogravimetric analysis (TGA) and Fourier Transform Infrared Spectroscopy (FTIR) to evaluate the regeneration energy required for CO2-loaded 3 M aqueous potassium sarcosinate (K-SAR) solvent and crystalline methyl-glyoxal-bisiminoguanidine (MGBIG) sorbent. Based on calorimetric measurements, for aqueous K-SAR, the sensible heat amounts to 1.5 GJ/tCO2 and the enthalpy of desorption is estimated at 3.68 GJ/tCO2. The total energy of regeneration will also include the enthalpy of solvent vaporization, which will depend on the thermodynamic conditions and the level of regeneration. For MGBIG, the total regeneration energy required was measured at approximately 7.0 GJ/tCO2. FTIR measurements were used to observe CO₂ and H2O release during the thermal regeneration process. For K-SAR, CO₂ release is seen to take place at temperatures greater than 80 °C and, for MGBIG, the CO₂ release takes place at temperatures greater than 100 °C. The moderate temperatures required for K-SAR and MGBIG regenerations compared to other DAC solvents/sorbents suggest that low-cost sources of heat, such as geothermal heat, could potentially be used for regeneration to drive down the cost of direct air capture.

Stable and metastable crystal structures and ammonia dynamics in strontium chloride ammines.

Metal halide ammines are promising ammonia storage materials due to their high ammonia densities and suitable decomposition properties. Here, we studied the polymorphism of ammines with a general formula of Sr(NH3)nCl2 (n = 1, 2, 4, 6, and 8) by combining the Fast and Flexible CrystAl Structure Predictor (FFCASP) with density functional theory (DFT) calculations. Furthermore, the lattice stability and the minimum energy paths for bulk and surface diffusion of NH3 were investigated by performing phonon and nudged elastic band (NEB) calculations. In addition to the successful reproduction of the reported experimental crystal structures of octammine (Pnma (IT number 62)), diammine (Aem2 (IT number 39)) and monoammine (Cmcm (IT number 63)), several isoenergetic polymorphs for each phase were also found. Not only the experimentally determined octammine and monoammine structures, but also the proposed structures for the hexammine and tetrammine phases were found to be metastable. While phonon calculations show instability for the experimental diammine structure, some of the proposed structures for the diammine phase showed thermodynamical stability. Moreover, NEB paths examining the bulk and surface diffusion of NH3 are in accordance with the experimental desorption enthalpies.

Multiple equilibria description of type H1 hysteresis in gas sorption isotherms of mesoporous materials

We report argon adsorption/desorption isotherms of MCM-41 and of SBA-15. The shape of all hysteresis loops we have observed corresponds to type H1. The data have been analyzed quantitatively using the multiple equilibria description and applying the notion of metastable thermodynamic equilibrium. It is remarkable and important that for both mesoporous materials the desorption process can be understood according to this description but with a corresponding equilibrium constant. This procedure therefore allows determining the thermodynamic values for the enthalpy and the free enthalpy of cavity desorption and thus to obtain information not available so far. We observed that the MCM-41 adsorption isotherms show first an increase with the characteristic Langmuir shape, followed by the almost instantaneous filling of cavities that ends as soon as all cavities are completely filled. The SBA-15 isotherms show a characteristic Langmuir shape at low relative pressure. The pressure slightly below the inflection point at prel, infl = 0.312 marks the beginning of the growth of a second layer, before an almost instantaneous filling of cavities takes place. It is interesting to observe that the values of the enthalpy and the free enthalpy for cavity filling and cavity desorption differ by about 0.1 kJ/mol for both, MCM-41 and SBA-15. This means that the driving force for developing a hysteresis is small. The hysteresis loop is therefore driven by delicate changes occurring within the cavities, partially or completely filled by the adsorbate. The monolayer coverage and monolayer desorption processes are decoupled from the cavity filling and cavity emptying processes. Knowing thermodynamic parameters for the hysteresis loop ultimately helps to better characterize and understand the experimental observations.

Broadly Tunable Atmospheric Water Harvesting in Multivariate Metal-Organic Frameworks.

Development of multivariate metal-organic frameworks (MOFs) as derivatives of the state-of-art water-harvesting material MOF-303 {[Al(OH)(PZDC)], where PZDC2- is 1H-pyrazole-3,5-dicarboxylate} was shown to be a powerful tool to generate efficient water sorbents tailored to a given environmental condition. Herein, a new multivariate MOF-303-based water-harvesting framework series from readily available reactants is developed. The resulting MOFs exhibit a larger degree of tunability in the operational relative humidity range (16%), regeneration temperature (14 °C), and desorption enthalpy (5 kJ mol-1) than reported previously. Additionally, a high-yielding (≥90%) and scalable (∼3.5 kg) synthesis is demonstrated in water and with excellent space-time yields, without compromising framework crystallinity, porosity, and water-harvesting performance.

Efficient SO2 removal using aqueous ionic liquid at low partial pressure

The development of novel absorbents is essential for SO2 removal. In this study, a novel ionic liquid (IL, [BHEP][HSO4]) was prepared, and water was selected as the co-solvent. The density and viscosity of aqueous [BHEP][HSO4] were measured and the SO2 absorption performance was systematically investigated. Furthermore, the thermodynamic properties of SO2 in aqueous [BHEP][HSO4] were calculated. Additionally, the mechanism of SO2 absorption in aqueous [BHEP][HSO4] was confirmed using Fourier-transform infrared and nuclear magnetic resonance spectroscopy. It showed that [BHEP][HSO4] absorbed 0.302 g⋅g−1 (g SO2/g IL) at an SO2 partial pressure of 2000 μl⋅L−1 at 303.2 K, and the SO2 desorption enthalpy was −39.63 kJ⋅mol−1. The mechanistic study confirmed the chemical absorption of SO2 in aqueous [BHEP][HSO4].

The location of adsorbed water in pigmented epoxy-amine coatings

Pigmentation can exacerbate problematic water uptake into protective paints, leading to detrimental effects such delamination, swelling and plasticization. Potential underlying mechanisms include poor adhesion between the pigment and binder, the formation of hygroscopic interphase regions close to the pigment particles, and internal stress. Here, we demonstrate the use of nanoscale chemical mapping using AFM-IR and nanothermal analysis to investigate the contribution of these mechanisms directly for model coatings based on diglycidyl ether of bisphenol-A (DGEBA) cross-linked with triethylenetetramine (TETA). Whilst moisture uptake increases in the presence of a microscale silica matting agent, no chemical interphase could be detected close to the wax-coated particles. Instead, nanothermal analysis reveals attenuated thermal expansion of the polymeric binder and raised Tg values, supporting favourable polymer-pigment interaction and an increase in internal stress in the polymeric network. This is confirmed by mapping water distributions directly using AFM-IR, where no accumulation at the intact pigment-polymer interface/interphase is detected. Finally, further evidence is provided by bulk thermal analysis, where increased plasticization and desorption enthalpies demonstrate that water sorption into the bulk polymer binder increases as a function of added pigment volume concentrations.

Hydrogen release mechanisms of MgH2 over NiN4-embedded graphene nanosheet: First-principles calculations

In this study, first-principles calculations were performed to investigate the catalytic effect of NiN4-G on the dehydrogenation of MgH2. Side-on MgH2 can be adsorbed stably on the NiN4-G monolayer and is preferentially adsorbed on the NiN4 site compared with the graphene site. The hydrogen desorption process, in which MgH2 dissociated to the Mg atom on the NiN4 site or graphene site and an H2 molecule in the vacuum, should overcome lower barriers than pure MgH2. This is because the corresponding Mg–H bond is weakened owing to the electron transfer between the Mg atom and the substrate. The hydrogen desorption enthalpies of the (MgH2)5 cluster on the NiN4 active and graphene sites are significantly smaller (0.11 eV and 1.50 eV, respectively) when H2+H2 is released from the cluster compared with those of the undoped MgH2 cluster (2.48 eV). Therefore, the NiN4-G monolayer can provide the double effect of the NiN4 active and graphene sites on improving the dehydrogenation performance of MgH2.

Screening ionic liquids for developing advanced immobilization technology for CO2 separation.

Developing immobilized-ionic liquids (ILs) sorbents is important for CO2 separation, and prior theoretically screening ILs is desirable considering the huge number of ILs. In this study, the compressibility of ILs was proposed as a new and additional index for screening ILs, and the developed predictive theoretical model, i.e., electrolyte perturbed-chain statistical associating fluid theory, was used to predict the properties for a wide variety of ILs in a wide temperature and pressure range to provide systematic data. In screening, firstly, the isothermal compressibilities of 272 ILs were predicted at pressures ranging from 1 to 6,000 bar and temperatures ranging from 298.15 to 323.15 K, and then 30 ILs were initially screened. Subsequently, the CO2 absorption capacities in these 30 ILs at temperatures from 298.15 to 323.15 K and pressures up to 50 bar were predicted, and 7 ILs were identified. In addition, the CO2 desorption enthalpies in these 7 ILs were estimated for further consideration. The performance of one of the screened ILs was verified with the data determined experimentally, evidencing that the screen is reasonable, and the consideration of IL-compressibility is essential when screening ILs for the immobilized-IL sorbents.

Influence of Poly-3-hydroxybutyrate Micro-Bioplastics and Polyethylene Terephthalate Microplastics on the Soil Organic Matter Structure and Soil Water Properties.

Adverse effects of microplastics on soil abiotic properties have been attributed to changes in the soil structure. Notably, however, the effects on the supramolecular structure of soil organic matter (SOM) have been overlooked, despite their key role in most soil properties. This work accordingly investigated the influence of plastic residues at various concentrations on the SOM supramolecular structure and soil water properties. To model plastic residues of micro-bioplastics, spherical or spherical-like poly-3-hydroxybutyrate (PHB) was used, while polyethylene terephthalate (PET) was used as a model of conventional microplastics. The results suggest that both types of plastic residues affect SOM properties, including physical stability (represented by water molecule bridges), water binding (represented by decreased desorption enthalpy or faster desorption), and the stability of SOM aliphatic crystallites. The results further showed that the polyester-based microplastics and micro-bioplastics affected the SOM abiotic characteristics and that therefore the observed effects cannot be attributed solely to changes in the whole soil structure. Notably, similar adverse effects on SOM were observed for both tested plastic residues, although the effect of PHB was less pronounced compared to that of PET.

Microstructure characteristics, hydrogen storage thermodynamic and kinetic properties of Mg–Ni–Y based hydrogen storage alloys

In this paper, the Mg95-X-Nix-Y5 (x = 5, 10, 15) alloy were prepared by vacuum induction melting. The X-ray diffraction was used to analytical phase composition in different states, and the Scanning Electron Microscope and Transmission Electron Microscope were used to characterize the microstructure and crystalline state. Meanwhile, the kinetic properties of isothermal hydrogen adsorption and desorption at different temperatures also were tested by the Sievert isometric volume method. The results indicate that the hydrogenated Mg–Ni–Y samples is a nanocrystalline structure consists of MgH2, Mg2NiH4, and YH3 phases. And, the in-situ formed YH3 phase not decompose in the process of dehydrogenation and evenly dispersed in the mother alloy, which plays a paly a positive the catalytic role for the reversible cyclic reaction of Mg and Mg2Ni phases. In addition, the Ni elements are effectively to improve the thermodynamic properties of the Mg-based hydrogen storage alloy, the desorption enthalpy of the Ni5, Ni10, and Ni15 samples successively decrease to 84.5, 69.1, and 63.5 kJ/mol H2. The hydrogen absorption and desorption kinetics of the Mg–Ni–Y alloy are improved obviously with the increase of Ni content, especially for Mg80Ni15Y5 alloy, which the optimal hydrogenated temperature is reduced to 200 °C, and the 90% of the maximum hydrogen storage capacity can be absorbed within 1 min, about 5.4 wt % H2. Besides, the dehydrogenated activation energy of the Mg80Ni15Y5 alloy also is reduced to 67.0 kJ/mol, and it can completely release hydrogen at 320 °C within 5 min, which is almost reached the hydrogen desorption capability of Ni5 alloy at 360 °C. This means that Ni element is a very positive element to reduce the hydrogen desorption temperature.

Hydrogen storage properties of V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy

In this paper, we present the synthesis, first hydrogenation kinetics, thermodynamics and effect of cycling on the hydrogen storage properties of a V0.3Ti0.3Cr0.25Mn0.1Nb0.05 high entropy alloy. It was found that the V0.3Ti0.3Cr0.25Mn0.1Nb0.05 alloy crystallizes in body-centred cubic (BCC) phase with a small amount of secondary phase. The first hydrogenation is possible at room temperature without incubation time and reaches a maximum hydrogen storage capacity of 3.45 wt%. The pressure composition isotherm (P–C–I) at 298 K shows a reversible hydrogen desorption capacity of 1.78 wt% and a desorption plateau pressure of 80.2 kPa. The capacity loss is mainly due to the stable hydride with the desorption enthalpy of 31.1 kJ/mol and entropy of 101.8 J/K/mol. The hydrogen absorption capacity decreases with cycling due to incomplete desorption at room temperature. The hydrogen absorption kinetics increases with cycling and the rate-limiting step is diffusion-controlled for hydrogen absorption.

Microstructure and electrochemical performance of Zn-doped of Mg2Ni1-xZnx hydrogen storage alloys

The microstructures, electrochemical, thermodynamic properties and desorption kinetics of as-cast Mg2Ni1-xZnx (x = 0, 0.08, 0.17, 0.25, 0.33, or 0.41) hydrogen storage alloys are investigated in this study. The X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that the Mg2Ni1-xZnx alloys are comprised of multiphase structure, thereinto, the diffraction peak of the major phase Mg2Ni is shifted to a small angle, and its unit cell volume is increased obviously. It was found that the maximum discharge capacity of the alloy electrode firstly increases and then decreases with the increase of the Zn content, and is 52.22 mAh/g when the x = 0.25. The PCT curves showed that the equilibrium hydrogen pressure of alloys is increased with increasing Zn content, which is mainly caused by the reducing of thermodynamic stability for the metal hydride by addition of Zn. Mg2Ni0.67Zn0.33 alloy has the lowest value of hydrogen desorption enthalpy (ΔH) and entropy (ΔS), which are 66.5 kJ/mol and 111.5 J/K/mol, respectively. It is also found that the addition of Zn in the Mg2Ni alloys significantly reduced their dehydrogenation activation energy (Ea). The Mg2Ni0.75Zn0.25 alloy has the lowest Ea value (17.01 kJ/mol), which is much lower than that (46.07 kJ/mol) of the free Zn Mg2Ni. This result is confirmed and in good agreement with the hydrogen diffusion rate (5.3068 cm2/s). However, the surface of the Zn-doped alloy is more easily corroded, leading to reduced capacity retention rate.

A Novel Strategy to Enhance Acid Strength of Zeolites by Incorporating Ge into Zeolite Framework

AbstractIn the long history of zeolite research, further enhancement of their acid strength has looked difficult. In this study, we achieved the enhancement by incorporating Ge into the zeolite framework. We synthesised the zeolites by a dry gel conversion method and characterised them in terms of morphology, crystal structure, and composition. We determined the acid strength of these zeolites by calculating the change in the enthalpy of desorption in the temperature‐programmed desorption of ammonia and confirmed that the acid strength of the zeolites with Ge was higher than that of a conventional zeolite. This enhancement of acid strength probably originated from the unprecedented interactions between Al and Ge, unlike the conventional interactions between Al and Si. The enhanced acid strength led to high efficiency of ethanol‐to‐propylene reactions, affording higher yield of propylene than that obtained using conventional zeolites.