The crystallization process of SAPO-34 molecular sieve with Si distribution in the crystals using diethylamine (DEA) as the template was investigated by XRD, SEM, IR, NMR, XRF, XPS and EDS ...techniques. It was found that the solution-mediated transport mechanism occurred during the crystallization, though the gel transformation cannot be completely excluded in the initial crystallization (
t
⩽
1
h). Si directly participated in the crystallization in the initial stage and incorporated into the framework of SAPO-34 by SM2 (P substitution by Si) and SM3 (pairs of Al and P substitution by 2Si) mechanisms. XPS analysis revealed the enrichment of Si on the surface of crystals. Based on the experimental results, a model of Si distribution in the crystals was proposed, which described a non-uniform distribution of Si in the crystals, with the increasing content from the core to the surface.
SAPO-34 and MeAPSO-34s (Me
Co, Mn, Fe) molecular sieves have been synthesized and used as catalysts for chloromethane transformation to light olefins. The influences created by metal incorporation ...are characterized with XRD, XRF, SEM, NMR, TG and H
2-TPR. The synthesized MeAPSO-34s have the same CHA topology structure, while metal incorporation gives rise to the increase of unit cell parameter and crystalline particle size. The coexistence of metal species in the synthesis starting gel has effect on the Si substitution into AlPO framework. Co, Mn or Fe incorporation generates a negligible difference on the chemical shift in
31P and
27Al MAS NMR.
29Si MAS NMR study has demonstrated that metal incorporation favors the Si island formation, predicting the stronger acidity. The reducibility of metal species in the synthesized MeAPSO-34s has been investigated by H
2-TPR with the comparison of metal-impregnated SAPO-34. Two weight losses in TG analysis from template decomposition in diluted oxygen suggest different chemical location of template molecules in the molecular sieves. For MeAPSO-34, more template removal occurrence at high temperature range indicates stronger template-framework interaction and stronger acidity than SAPO-34 after calcinations. All the SAPO-34 and MeAPSO-34 molecular sieves are very active and selective catalyst for light olefins production. Metal incorporation improves the catalyst life and favors the ethylene and propylene generation. These catalytic properties enhancements are possibly related to the mechanism of chloromethane conversion with deposited coke species as reaction center.
A series of MgAPSO-34s with various Mg stoichiometries were synthesized, characterized by multiple techniques such as, powder XRD, solid state MAS NMR, DR UV–vis spectrometry, NH
3-TPD and FTIR and ...used as catalysts in the direct conversion of chloromethane into light olefins. XRD results revealed that the unit cell parameters of the CHA structure increased with Mg incorporation. The appearance of some new peaks in the
31P MAS NMR spectra of MgAPSO-34 indicated new P co-ordination states as a result of the substitution of Al by Mg. Mg incorporation also modified the Si co-ordination states. A DR UV–vis spectroscopic study evidenced that, in comparison with an MgO crystal, the Mg species in the MgAPSO-34s were quite different. These changes in crystal structure and the chemical environment of framework elements created by Mg substitution modified the acidity and catalytic properties. The acidity of SAPO-34 and MgAPSO-34 determined with NH
3-TPD and FTIR techniques showed that the amount of acid sites, especially those of strong acidity, is reduced with the increase of Mg incorporation. Direct chloromethane transformation was carried out over SAPO-34 and MgAPSO-34. All the samples were very active and selective catalysts for light olefin production from chloromethane. Compared with SAPO-34, MgAPSO-34 showed an enhanced conversion, longer catalyst life and improved light olefin selectivity.
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► DNL-6, a novel SAPO molecular sieve with RHO topology has been synthesized. ► High thermal/hydrothermal stability, large micropore surface area and pore volume. ► Solution-mediated ...transport mechanism. ► Si content in crystals increased gradually with the crystallization time.
A new silicoaluminophosphate molecular sieve with the RHO framework (designated as DNL-6, Dalian National Laboratory Number 6) has been hydrothermally synthesized for the first time with diethylamine (DEA) template in the presence of cetyltrimethylammonium bromide (CTAB). Influence factors, such as CTAB amount and silica content in the starting gel, crystallization time and temperature, were investigated. It was observed that DEA together with CTAB were necessary for the formation of pure RHO phase, which appeared in a narrow range of the starting gel compositions and synthesis conditions. Only DEA and water molecules were found in the as-synthesized DNL-6 without any existence of CTAB. All Si atoms incorporated into the framework with Si(4Al) environment at a high amount close to the theoretical maximum value. DNL-6 molecular sieve possessed high micropore surface area (777
m
2/g) and large micropore volume (0.36
cm
3/g) with good thermal and hydrothermal stability. Crystallization course was studied by monitoring solid products at different crystallization times with XRD, SEM, XRF, NMR and CHN elemental analysis. It is proposed that DNL-6 crystals were more likely formed directly from the crystallization of liquid phase rather than from the transformation of amorphous materials, and Si atoms directly incorporated into the DNL-6 framework from the very beginning of the crystallization via a SM2 mechanism.
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•Effect of concentration models on fluidized bed measurements was studied.•Frequency dominant properties of bubble are unrelated to concentration models.•Selection of bubble threshold ...is strongly related to concentration models.•A general concentration model given in formula of a power function is proposed.
Electrical capacitance tomography (ECT) has been widely applied in gas-fluidized bed measurements, which however is constrained by the sensor designs, image reconstruction algorithms and concentration models owing to the “soft-field” nature. Among the three factors, the concentration models that map the permittivity to solid concentration distribution are less understood. In this paper we present an experimental investigation on the effects of concentration models for the ECT measurements of gas-fluidized beds containing Geldart’s group A particles. The results show that the bubble velocity, bubbling frequency and fluidization regime transition velocity are intrinsic whatever the concentration model employed. However, the time-averaged solid concentrations measured by ECT with the conventional concentration models are usually higher than the corresponding results obtained by the bed expansion experiments in homogeneous fluidization. It has been further found that the concentration models can significantly influence the threshold selection in identifying the bubbles and surrounding emulsion phase in gas-fluidized beds. Thus, a new concentration model given in the formula of a power function (ϕ=Gα) has been proposed, with the optimal exponent α obtained via the least-square fitting of solid concentration in bed expansion during homogeneous fluidization. The model is robust and the sensitivity analysis shows that the variation of time-averaged solid concentration is negligible (≤ 3%) suppose that α changes by ± 20%.
MnAPSO-34 molecular sieve has been synthesized with triethylamine as the template, characterized with XRD, XRF,
31P,
27Al and
29Si NMR and FT-IR techniques and compared with SAPO-34. The template ...decomposition and removal have been investigated with TG–DTG–DSC coupled with mass spectrometer. Mn incorporation generates a negligible difference on the chemical shift in
31P and
27Al MAS NMR, while an effect on the intensity of resonance peaks is revealed.
29Si MAS NMR study has demonstrated that Mn incorporation favors the Si island formation, which may give rise to the stronger acidic sites. The thermal analysis (TG–DSC) on template removal in a diluted oxygen atmosphere, leading to the formation of CO
2, NO and H
2O, showed, besides a low temperature endothermic weight loss due to the desorption of water, two weight losses (200–400 and 400–600
°C) for SAPO-34 and MnAPSO-34, suggesting two different chemical location environments of template molecules in these two molecular sieves. The quantity of template removed at higher temperature range is much higher in MnAPSO-34, indicating stronger template–framework interaction and stronger acidity after calcination. The acid difference caused by Mn incorporation has also been evidenced by ammonia adsorption evaluated by FT-IR. Chloromethane transformation was carried out over MnAPSO-34 and SAPO-34 and the catalytic performance showed that both molecular sieves are very active and selective catalyst for light olefins production. MnAPSO-34 demonstrated higher activity and light olefins selectivity.
A ZSM-5-based catalyst was prepared by spray-dry method for fluidized-bed naphtha catalytic cracking. Multi-techniques, such as X-ray diffraction, scanning electron microscope,
27
Al MAS NMR, and NH
...3
–TPD, were employed for the investigation of ZSM-5 framework stability, framework dealumination, and catalyst acidity variation in hydrothermal treatment. Catalytic performances of fluidized-bed naphtha catalytic cracking at 630–680 °C indicated that light olefins and other value-added products could be more efficiently produced compared with the commercial process of thermal steam cracking. Long-term catalytic evaluation implied that naphtha catalytic cracking over the catalyst prepared with spray-dry method and hydrothermal treatment can be carried out at a variable reaction condition with a relatively high and stable light olefins yield.
This article presents investigation of 3-D electrical capacitance tomography (ECT) sensors with different excitation strategies, including dual-electrode excitation in the same plane, in adjacent ...plane, in planes separated by one plane or two planes, and in the adjacent position or opposite position in the same or different planes. The results show that a 3-D ECT sensor with dual-electrode excitation in the opposite position provides higher mean capacitance and lower standard deviation than in the adjacent position. Dual-electrode excitation in different planes can increase the capacitance signal and decrease the dynamic range of capacitance measurements, and thus, decrease random errors and improve the quality of 3-D reconstructed images. A 3-D ECT sensor with dual-electrode excitation in planes separated by one plane and opposite position can provide small measurement uncertainty and high image quality.
The transition of the fluidization behavior of Geldart B particles to that of A particles induced by temperature change was investigated by a developed high-temperature electrical capacitance ...tomography (ECT) sensor. Silica particles with a Sauter mean diameter of 237 μm and density of 2650 kg/m3, typically Geldart B particles under ambient conditions, were fluidized in a column of 5.5 cm from 20 to 600 °C. With the increase in temperature, ECT measurements showed a decrease in minimum bubbling velocity (U mb), no bed expansion characteristic in the homogeneous fluidization regime, an absence of multiple-bubbles regime, and a larger bubble size. The pressure drop against superficial gas velocity curves at elevated temperatures confirmed homogeneous fluidization between the minimum fluidization velocity (U mf) and U mb. Our analysis demonstrates that cohesive interparticle forces, which increase linearly with temperature, are responsible for the fluidization behavior transition of the silica particles.
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