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It has been demonstrated that a novel p-xylenediamine/methanol soaking method could efficiently remove the polymer--zeolite interface defects of the mixed-matrix structure. In this work, the mixed-matrix structure is in the form of an ultrathin (1.5-3 µm) polysulfone/ zeolite beta mixed-matrix layer that is supported by a neat Matrimid® layer in dual-layer composite hollow fibers. The particle's loading in this thin layer has reached 30 wt %. The ideal selectivities of the mixed-matrix hollow fibers (30 wt % of zeolite)for O2/N2 and CO2/CH4 separation were roughly 30 and 50% superior to that of the neat PSF/Matrimid® hollow fibers, respectively. Investigation of the morphology of the mixed-matrix selective layer and its relation with gas separation performance indicated that without p-xylenediamine/methanol solution treatment, the outer layer showed various polymer--zeolite interface structures in different fibers with the same heat treatment procedures; this situation might lead to the different selectivities after coating. However, by applying p-xylenediamine/methanol processing on the fibers before thermal treatment, the fibers obtained a more uniform structure and improved attachment between polymer matrix and zeolite surface. Hydrogen bonding was proposed as the possible mechanism for the tighter attachment between the two phases. The improvement of separation efficiency was presumably related to the polymer chain rigidification, partial pore blockage, and/or favorable interaction between the gas penetrants and zeolite framework. [ABSTRACT FROM AUTHOR]

Among the tested carbons, only carbon aerogels (CAs) had significant effects on the hydrogen desorption behavior of LiAlH4. Adding 15 wt% CAs decreased the desorption temperature, while, with 5 wt% CAs, the desorption temperature was still the same. The desorption kinetics of the hydride with 15 wt% CAs, however, did not improve but adding 5 wt% CAs resulted in desorption kinetics improvement. In addition, mixing LiAlH4 with a catalyst decreased the desorption temperature and desorption kinetics of the hydride. [ABSTRACT FROM AUTHOR]

SUMMARY TiCl3 has been considered as the best catalyst for the hydrogen desorption/re-absorption of NaAlH4 in terms of kinetic enhancement. However, the formation of NaCl as a by-product leads to the decrease in the reversible hydrogen capacity of NaAlH4. In this work, TiO2 and metallic Ti were selected as catalysts for the reaction to avoid the formation of the by-product. The comparison of the catalytic activity of Ti, TiCl3, TiO2 and Ti(OBu)4 on the hydrogen desorption/absorption NaAlH4 were carried out. It was found that TiO2 doped NaAlH4 exhibits similar behavior as TiCl3 doped NaAlH4 with the reversible hydrogen capacity about 3.8 wt% (H/M). In addition, TiO2 doped NaAlH4 exhibits the superior hydrogen re-absorption rate to the one doped with TiCl3. That may be due to the Ti3+ defect sites on the surface of TiO2 would facilitate the hydrogen dissociation. Moreover, high surface area of TiO2 prevents the segregation and the morphological change of the desorbed substances (NaH and Al). This benefits to the mass transfer into the hydride system. However, doping with TiO2 also produces sodium oxide and hydroxide as by-products. Unexpectedly, metallic Ti doped NaAlH4 shows the lowest hydrogen desorption/re-absorption among the tested samples. Its hydrogen reversible capacity is around 1 wt% (H/M). The formation of TiHx (1 < x < 2) was detected in the sample after the hydrogen desorption/reabsorption. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: The hydrogen desorption/absorption of the 2:1 mole ratio of LiBH4 and MgH2 with and without a catalyst was investigated. In the case of the uncatalyzed LiBH4/MgH2 mixture, the first hydrogen desorption started at 50°C. The amount of released hydrogen was 8.6wt% at the first hydrogen desorption and dramatically reduced to 1.8wt% at the tenth hydrogen desorption. The decrease in the hydrogen capacity in the subsequent hydrogen desorption may be due to the irreversibility of LiBH4. To investigate effects of a catalyst on the hydrogen desorption, 3mol% of TiCl3, HfCl4, ZrCl4, or VCl3 was added to the LiBH4/MgH2 mixture. The lowest hydrogen desorption temperature, 260°C, was from the sample with TiCl3. An amount of the catalyst also influenced the kinetics of the hydride mixture and 5mol% seems to be an optimum amount of TiCl3 that resulted in the lowest hydrogen desorption temperature, 240°C. In addition, the higher the amount of a catalyst, the lower the amount of the released hydrogen. [Copyright &y& Elsevier]

Abstract: The segregation of bulk Al after the hydrogen desorption may be one of many reasons why there is the incomplete hydrogen re-absorption and the need for higher desorption temperature in subsequent desorption of HfCl4- and TiO2-doped NaAlH4. In this work, we attempted to improve hydrogen storage capacity of 4mol% HfCl4- and 4mol% TiO2-doped NaAlH4 by adding a co-dopant (graphite, activated carbon, and MCM-41) using ball milling technique. It was found that the co-dopants significantly affect the hydrogen desorption/re-absorption of the hydride. Results show that the hydrogen re-absorption capacity of HfCl4- and TiO2-doped NaAlH4 added with the co-dopants increases 10–50% as compared with that without a co-dopant, and graphite seems to be the best co-dopant. Moreover, the hydrogen desorption temperature in the subsequent cycle of co-dopant doped samples decreases about 10–15°C compared to the sample without a co-dopant. It could be explained that high porosity and large surface area of the co-dopants would decrease the segregation of bulk Al after the desorption and improve hydrogen diffusion in/out bulk of desorbed/re-absorbed samples. [Copyright &y& Elsevier]

The main objective of this work was to investigate the effect of two transition metals (0–9mol% ZrCl4 and HfCl4) on hydrogen desorption/absorption of NaAlH4 and LiAlH4. The hydrogen desorption was carried out over a wide temperature range of 25– while the hydrogen absorption took place at and between 50– for NaAlH4 and LiAlH4, respectively, with the same pressure of 11MPa. The result revealed that the transition metals (Zr and Hf) could improve the kinetics of the hydrogen desorption on NaAlH4. NaAlH4 doped with HfCl4 released hydrogen at the lower temperature than that of ZrCl4-doped NaAlH4. In addition, the rate of hydrogen desorption increased with increasing the amount of HfCl4 doping. However, the maximum hydrogen capacity of was obtained from the first desorption, and dropped to 2.2–2.6wt% in the subsequent cycles. This may be because of, after the hydrogen desorption, the hydrides melt due to the high temperature resulted in agglomeration of the hydrides and segregation of Al. XRD results showed the formation of NaCl after milling of NaAlH4 with 4mol% HfCl4 and no evidence of any Hf-containing phase. However, with 10mol% HfCl4, the formation between Al and Hf in the form of Al3Hf was observed. This compound may act as catalyst in the reversible hydrogen desorption/absorption. For LiAlH4, ZrCl4 or HfCl4 also enhanced the kinetics of desorption of LiAlH4. Moreover, it was observed that LiAlH4 released hydrogen during the milling. After the hydrogen desorption from LiAlH4, no hydrogen absorption was observed for the undoped hydride or that doped with HfCl4 or ZrCl4. This may be because of the instability of LiAlH4. Moreover, milling and dopant may also destabilize the structure of LiAlH4 causing the irreversibility of hydrogen desorption/absorption on LiAlH4. [Copyright &y& Elsevier]

Abstract: The main objective of this work was to investigate the different effects of transition metals (TiO2, VCl3, HfCl4) on the hydrogen desorption/absorption of NaAlH4. The HfCl4 doped NaAlH4 showed the lowest temperature of the first desorption at 85°C, while the one doped with VCl3 or TiO2 desorbed at 135°C and 155°C, respectively. Interestingly, the temperature of desorption in subsequent cycles of the NaAlH4 doped with TiO2 reduced to 140°C. On the contrary, in the case of NaAlH4 doped with HfCl4 or VCl3, the temperature of desorption increased to 150°C and 175°C, respectively. This may be because Ti can disperse in NaAlH4 better than Hf and V; therefore, this affected segregation of the sample after the desorption. The maximum hydrogen absorption capacity can be restored up to 3.5wt% by doping with TiO2, while the amount of restored hydrogen was lower for HfCl4 and VCl3 doped samples. XRD analysis demonstrated that no Ti-compound was observed for the TiO2 doped samples. In contrast, there was evidence of Al–V alloy in the VCl3 doped sample and Al–Hf alloy in the HfCl4 doped sample after subsequent desorption/absorption. As a result, the V- or Hf-doped NaAlH4 showed the lower ability to reabsorb hydrogen and required higher temperature in the subsequent desorptions. [Copyright &y& Elsevier]

Effects of tetrahydrofuran (THF) concentration on the mixed methane hydrate formation, dissociation, and stability were investigated. The experiment was conducted at 286.2 K and 6 MPa in a quiescent reactor. The presence of THF below 2.5 mol% did not show the evidence of hydrate formation. However, the concentration above 2.5 mol% enhanced the methane formation rate and the methane consumption. Increasing the THF concentration decreased the induction time as the result of the decrease in the surface tension. Moreover, the methane uptake and formation rate increased with the THF concentration due to the higher degree of hydrate nucleation. The methane recovery after the dissociation experiment showed up to 96%. Furthermore, the hydrate stability increased, and the hydrate dissociation kinetics decreased with the increase in the THF concentration. The optimum THF concentration to enhance and improve the hydrate formation kinetics and stability is its stoichiometric concentration.