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•Negative impact of the desilication of ZSM-5 on the propane aromatization.•Negative impact of the desilication of ZSM-5 on the cyclohexane dehydrogenation.•No impact of the ...desilication of ZSM-5 on the n-hexane cracking.•At higher gallium content, a steric retention of coke precursors in the mesopores.
Gallium containing ZSM-5 zeolites were prepared by ion exchange and mechanical mixture from the parent ZSM-5 and two desilicated zeolites to be tested in propane aromatization. The alkaline treatment was made by sodium hydroxide, with or without tetrabutylammonium hydroxide. The catalysts prepared from desilicated zeolites are less active in propane aromatization and cyclohexane transformation and less selective into aromatics than those prepared from the parent zeolite. The desilication did not affect the acid properties, especially the strength of acid sites as seen by CO adsorption at 77 K and n-hexane cracking.
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•Propane aromatization and cyclohexane transformation were carried at 803 K.•Gallosilicates are less active than ion exchange and mechanical mixture samples.•Activation of propane is ...by an alkyl mechanism.•Higher coke content is formed on gallosilicates.•The coke is toxic and localized in the micropores.
Three series of gallium-containing MFI zeolite were prepared with different gallium content by weight, by hydrothermal synthesis Ga-MFI (0.2–4.7 wt% Ga), ion exchange Ga/P (0.5–6.8 wt% Ga) and mechanical mixture Ga+P (0.5–10 wt% Ga). Under hydrogen at 600 °C on the aluminosilicate doped with gallium, a reducing solid ionic exchange occurs between a mobile species (Ga2O) and the protonic sites of the zeolite. The proportion of Ga implied in this exchange depends on both the proximity between Ga2O3 and the zeolite and the initial gallium content. The solid reductive exchange remains limited, ranging from 25% to only a few percent; therefore the gallium species content located in the zeolite micropores is only 0.2–0.4 wt%. The aluminosilicates doped with Ga are more active in both cyclohexane dehydrogenation and propane aromatization than the gallosilicates: the gallium in the framework is much less active than gallium in the exchange position.
The catalytic fast pyrolysis of oak over two zeolites (microporous and hierarchical) was investigated in a microfluidized bed reactor (MFBR) at 500 °C and as a function of the biomass-to-catalyst ...ratio. A hierarchical zeolite was produced by desilication with a NaOH solution of a conventional HZSM-5 zeolite. The outlet of the MFBR was connected to a single photoionization mass spectrometer (SPI-MS) for the on-line analysis of volatiles. This on-line analysis method allows studying the dynamics of volatile formation (in real time) and the deactivation of 2 zeolites upon stepwise injections of wood particles. Strikingly, the selectivity of targeted mono-aromatic compounds (quantified by gas chromatography) is doubled after desilication of the zeolite. The coked zeolites were characterized by TEM-EDX, digestion in fluoric acid, MALDI-TOF MS, etc . Three different types of coke are evidenced: (1) coke trapped inside micropores, (2) external coke formed on the outer surface of the crystals and (3) coke precursors deposited in the mesopores. The latter two cokes are much less toxic than the microporous coke. The open mesopores produced after desilication can be seen as “highways” where big molecules (such as levoglucosan) can diffuse to more accessible pore mouths. Brønsted acid sites present on microporous mouths can be active for the conversion of bigger molecules to small fragments. These fragments diffuse and form aromatics in the micropores (shape selectivity). The mesopores also promote the evacuation of catalytic products, thus enhancing the selectivity of mono-aromatic hydrocarbons. Desilicated zeolites present higher selectivity to mono-aromatics and stability upon coke deposition than microporous zeolites.
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•Incorporated HPW-SBA catalysts are more resistant toward deactivation in oxidative desulfurization of real fuel than impregnated ones.•Incorporated HPW-SBA catalyst showed enhanced ...interaction between supported phase and carrier as shown by Tof-SIMS analysis.•No deactivation was observed for 9 days on incorporated HPW-SBA catalyst in the ODS of a SRGO 2000 ppmS.•This preparation method appears promising to reduce deactivation in the ODS of real feeds.
Incorporated W-SBA based catalysts were prepared by a direct synthesis method involving phosphotungstic acic (HPW) together with mixed structure directing agents, and by incipient wetness impregnation for comparison purpose. In the oxidative desulfurization (ODS) of dibenzothiophene (DBT, 500–1500 ppmS) and of a Straight Run Gas Oil (SRGO, 2000 ppmS) in batch reactor, the impregnated solids showed higher efficiency than the incorporated ones, which was attributed to a better accessibility of the active phase present at the surface of the support in the impregnated catalysts. However, in continuous evaluation using a fixed bed reactor, the incorporated catalyst showed a stable conversion in the ODS of SRGO 2000 ppmS for 9 days while within 24 h the impregnated solid was almost completely deactivated. Higher interaction of the active phase with the support in incorporated catalysts was evidenced by Time-of-Flight Secondary Ions Mass Spectrometry (ToF-SIMS) analysis, which can be at the origin of less leaching of tungsten species during reaction and thus explain their better resistance towards deactivation. The lower efficiency of incorporated catalysts compared to impregnated ones in ODS reaction carried out in a batch reactor is clearly compensated by their resistance towards leaching of the active phase in a continuous test. These results highlight the importance of evaluating not only the catalysts performance but also their lifetime in conditions more representative of industrial ones.
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•CoMo HDS catalyst was active in ODS of SRGO, with an optimum Co/Mo ratio of 0.5.•Modification of CoMo catalysts by sulfones improved their HDS performance.•The presence of sulfones ...led to a redispersion of cobalt and to a better promotion.
Oxidative desulfurization reaction (ODS) of Straight Run Gas Oil (SRGO) was carried out on a CoMoP catalyst, classically used in hydrotreatment. Optimum Co/Mo ratio for the ODS reaction was found at 0.5, which also corresponds to an optimum ratio for HDS. This catalyst allowed to oxidize sulfur compounds to more polar sulfones which can be adsorbed on the catalyst and served as modifying agent. The catalyst before and after ODS and the CoMoP catalyst modified by a commercial dibenzothiophene sulfone (DBTO2) were tested in gas oil HDS and better performances were obtained for the modified catalysts for a sulfone content of 10wt%. This effect was attributed to the redispersion of cobalt species observed after modification, leading to more efficient promotion.
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•Effect of desilication on characteristics of micro- and nanocrystals *BEA zeolites.•Desilication treatments in NaOH alone and NaOH mixed with pore directing agents.•Catalytic ...performance of micro- and nanocrystals before and after desilication.•Effect of Pt content on activity and selectivity towards isomers products.•Effect of zeolite and catalyst’s properties on final activity and selectivity.
Two commercial nanocrystal *BEA zeolites CP811 (P1) and CP814E (P2) with same total Si/Al ratio (Si/Altotal=12) but different textural properties, were desilicated using different alkaline treatment; classical in presence of NaOH alone, or incorporated with a pore directing agent. P1 was desilicated by NaOH alone, and NaOH+TPABr, while P2 was desilicated in presence of NaOH+TBAOH. A synthesized microcrystal *BEA zeolite with Si/Al total ratio of 36, was also desilicated with NaOH alone, NaOH+TPABr, and NaOH+TBAOH. All parent and desilicated zeolites were transformed into bifunctional catalysts by platinum loading and were tested in the hydroisomerization of n-C10. P1 zeolite was impregnated with different Pt contents to insure maximum activity and selectivity towards isomers products. It has been seen that the improvement of the textural properties by desilication using the different pore directing agents, was not always the cause behind an increase or decrease in the activity and selectivity of the catalyst, but rather was more the location of the Pt particles and their predicted distance from the acidic sites. However, the presence of an interplay between the inter- and intracrystalline mesopores, with the additional parameter of Pt-H+ distance, serve better as combined characteristics behind the final activity and selectivity.
W based catalysts were synthesized by dry impregnation of SBA-15 mesoporous silica with phosphotungstic acid (HPW) solution with W contents between 5 and 20%, the HPW compound being preserved after ...calcination. The catalysts performance and the reactivity of various sulfide compounds were evaluated in the oxidative desulfurization (ODS) of model solutions and of real diesels, with sulfur contents ranging from 50 to 2000 ppm. The reactivity of benzothiophene and dibenzothiophene compounds was different in the ODS of model solutions but globally identical in the ODS of SRGO. The monitoring of the concentration of a range of alkyl DBT compounds (with alkyl groups from C2 to C5) in LGO confirmed the importance of the steric hindrance of alkyl substituents in the 4,6 position near the S atom, as well as of the size of the alkyl groups. Among the
W/SBA series, the catalyst with the highest loading showed the best performance in the ODS of LGO and SRGO while the catalysts efficiency could not be discriminated in the ODS of model solutions. In the ODS of both model solutions and real feeds, the W/SBA catalyst was found to be much more efficient than a catalyst obtained by impregnation of a commercial silica with similar loading, highlighting the beneficial use of a mesoporous support with high surface area and pore volume that allowed well-dispersed tungsten species to be obtained. The quantity of sulfones precipitated and/or retained on the catalyst depended on the feed and was found to be higher in the ODS of model solution than in the ODS of real feeds. The precipitated/retained sulfones on the support may induce catalyst deactivation, which highlights the importance of the textural properties of the support. This detailed study points out the difficulty of extrapolating results obtained in the ODS of model solution to the ODS of real feeds.
The desilication of two commercial nano- and one synthesized microcrystal *BEA zeolites via different alkaline solutions have led to several observations. In the nanocrystal *BEA zeolites, the use of ...NaOH alone reduced the crystallinity and microporosity, which was recovered after the use of pore directing agents as tetrapropylammonium bromide (TPABr). The use of tetrabutylammonium hydroxide (TBAOH) was not as much effective with NaOH as TPABr in terms of crystallinity and microporosity recovery, as it wasn't also as much effective as was NaOH alone in introducing intracrystalline mesopores to these nanocrystals. In the microcrystal series, the use of NaOH with TBAOH was seen to be more effective than NaOH alone or NaOH with TPABr. The increase of relative Brønsted acidity was observed at lower alkaline concentrations with a pronounced decrease at higher alkaline media. The company of the pore directing agents was seen to provoke the formation of new Lewis acid sites. In the cracking of n-hexane, the desilication treatments were not seen to improve the catalytic performance of the requested catalysts, as the slight deactivation of the catalysts was at the basis of coke formation. The activity was seen to drop due to even the diffusional limitations occurring or due to loss of acidity after desilication. More olefin and isomers products were produced despite of coke formed which was considered non-toxic being located inside the mesopores.
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•Desilication of micro- and nanocrystals *BEA zeolites by alkaline treatments.•Desilication favored introduction of intracrystalline mesopores.•n-hexane cracking tests before and after generation of intracrystalline mesopores.•Monofunctional mechanism was always observed even in presence of mesopores.•Coke can be found non-toxic being trapped in the mesopores also.
The present paper highlights the influence of desilication of nanocrystal *BEA zeolites (CP811 and CP814E) by different alkaline treatments in presence of NaOH alone, NaOH + TPABr and NaOH + TBAOH, ...on the catalytic performance in the hydroisomerization reactions of
n-
alkanes (
n-
C
10
,
n-
C
12
and
n-
C
14
). The well-balanced catalyst was reached after impregnation of 1.5 wt% of Pt, where the activity and isomers selectivity was seen to be the maximum, knowing that the Pt content effect was studied on the CP811 zeolite catalyst. All the other catalysts were after impregnated by approximately 1.5 wt% of Pt. The improvement of the textural properties by means of desilication was not always accounting for the influence of the catalytic performance of the catalysts, but rather it may be the bifunctional characteristics in charge. The impact of chain length was investigated on the catalysts to study if the presence of the inter- and intracrystalline mesopores would account for better diffusion of larger molecules as,
n-
C
12
and
n-
C
14
. It was found on the majority of the catalysts that the activity was high whether the chain length was, but the isomers selectivity was decreasing with chain length except on one catalyst that possesses high textural and bifunctional characteristics. Among the three
n-
alkanes studied,
n-
C
12
have marked the highest TOF values and lowest selectivity to isomers, a phenomenon attributed to the confinement effect that seems to increase the interaction of
n-
C
12
molecules with the acidic sites of the zeolites, apparently causing their strength to be higher. This effect was pronounced more with
n-
C
12
than the other two n-alkanes.
Graphical Abstract
Sodium chalcogenide compounds, glassy and crystalline, are promising materials for a number of potential applications in non-linear optics (NLO), photovoltaic, and energy conversion and storage. This ...work deals with the study of the newly synthesized glasses in the Na2Se–As2Se3 chalcogenide system as well as the corresponding ternary crystals, NaAsSe2 and Na3AsSe3. We examine the glass-forming domain in the pseudo-binary system using melt-quenching and mechanical milling techniques. We also analyse the macroscopic and electrical properties of the glasses in comparison with their crystalline counterparts, and the annealed glasses yielding glassy/crystalline alloys.
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