The oxidative cracking of
n
-decane,
n
-dodecane,
n
-tetradecane and
n
-hexadecane was assessed at millisecond contact times on a platinum catalyst at a C/O molar ratio of 1.8. The thermal mapping of ...the reactor showed the similar thermal behaviour of the millisecond reactor at the steady-state for all reactants, corresponding to the autothermal operation of the combustion catalyst in the range of 440-550 °C. The heat provided led to a thermal gradient in the post-catalytic gas-phase and sustained the endothermic formation of valuable linear α-olefins (LAOs). The LAO yields increased with the carbon chain length of the reactant from 11.4 wt% starting from
n
-decane to 20.2 wt% starting from
n
-hexadecane. Furthermore, the LAO carbon chain length tends to be higher when feeding the MSR with larger
n
-alkanes. Two mathematical models are able to predict the distributions of organic products: (i) the Anderson-Schultz-Flory law for cracking products,
i.e.
LAO and
n
-aldehydes and (ii) a model based on the hydrogen atom abstraction probability for oxygenated compounds, the formation of which does not involve carbon-carbon bond cleavage,
i.e.
tetrahydrofurans, oxiranes and ketones.
The oxidative cracking of
n
-decane,
n
-dodecane,
n
-tetradecane and
n
-hexadecane was assessed at millisecond contact times on a platinum catalyst at a C/O molar ratio of 1.8.
•First report on the synthesis of hierarchical zeolite templated carbons.•Tailored microporous topology with high degree of homogenous intraparticular mesoporosity.•Mesoporosity from steamed zeolites ...was meticulously transcribed to the ZTC particles.•The rearrangement of surfactant-templated mesoporosity was observed.•Surfactant-templating of USY produces amorphous phase that fills-up steamed mesoporosity.
The achievement of hierarchical zeolite templated carbons (ZTCs) is firstly reported through the use of mesoporous zeolites as sacrificial scaffolds. The achieved hierarchical ZTCs feature both tailored microporosity and high degree of mesoporosity and allow hence to bridge the gap between classical ZTCs and CMK-like materials. Mesoporosity from steamed zeolites was meticulously transcribed to the ZTC particles. The use of zeolites featuring surfactant-templated mesoporosity allowed for achieving a ZTC with a very high mesopore volume (0.85 cm3 g−1). The rearrangement of surfactant-templated mesoporosity could be evidenced through nitrogen physisorption and transmission electron microscopy on ultramicrotomed samples. The rearrangement of mesoporosity during ZTC formation has been found to impact coherent crystal size and the lattice strain of the zeolite template. The combination of data from X-ray diffraction and nitrogen physisorption allowed further to reveal that the thickness of the ZTC skeleton is impacted by the degree of intracrystalline mesoporosity in the template zeolite. Through comparing the textural characteristics of the zeolite templates and of the achieved hierarchical ZTCs, important evidence was found that the surfactant-templating of USY zeolite leads to the development of a secondary amorphous phase and that the reduction of the microporous volume during this process is merely apparent.
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•*BEA morphology impacts coking and decoking in the ethanol dehydration.•Micron-, nanometer-sized and nano-sponge *BEA zeolites were synthesized.•Extreme crystal downsizing favors ...products mass transfer that limits coke growth.•A low amount of light coke in nano-sponge is enough to blocks micropores access.
The impact of crystal size and textural properties of *BEA-type zeolites in the conversion of ethanol-to-hydrocarbons at 350°C and under 3.0MPa pressure is investigated. Three *BEA zeolites (molar ratio Si/Al of ca. 23) with important differences concerning their textural properties are synthesized: a material constituted of micron-sized crystals and two hierarchical porous zeolites with a mesoporous network created either by the aggregation of nanometer-sized crystals or by the use of a gemini-type quaternary ammonium surfactant given a nano-sponge zeolite. The micrometric zeolite favors the formation of polyaromatic molecules in same proportion than the concentration of Brønsted acid sites. The deactivation mode of acid sites is poisoning rather than pore blocking. The short thickness of crystal in the nano-sponge zeolite combined with its low acidity, limit the growth of coke molecules as well as their accumulation inside micropores. Coke precursors (alkylbenzenes) are adsorbed on many silanol group located in the ordered mesopores, but the extreme downsizing of the zeolite crystal leads to an unexpected pore blocking by these light molecules.
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•n-C6 cracking: function of zeolite local environment of H+ (confinement effect).•Glycerol etherification: function of zeolite voids and interconnection volume.•Zeolite pore geometry ...impacts positively successive reactions.•Zeolite pore geometry leads to product auto-inhibition.
The role of acidity (nature, concentration, strength) and textural properties in the etherification of glycerol with tert-butyl alcohol was studied for a wide range acid catalysts, such as Amberlyst® 15, silica, alumina, silica alumina and four type of zeolite, i.e. FAU, MOR, ∗BEA and MFI. The etherification of glycerol by tert-butyl alcohol is a thermodynamically limited reaction that occurs through a successive reaction sequence and follows an Eley-Rideal type mechanism. We found major evidence that glycerol etherification is not only a function of the amount of Brønsted acid sites, but that it further proceeds via a product shape selectivity mechanism. Indeed, the formation of di-substituted ethers appears at very low conversions for zeolites compared to meso- and macroporous acid catalysts. ∗BEA and MFI zeolites feature similar confining voids and resulting thus in similar intrinsic acid strengths (as proved by n-hexane cracking), but differ in the connectivity (4 vs. 6 channels) and access to these voids (0.54 vs. 0.67 nm), which leads to diffusion issues, notably for the MFI zeolite.
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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.
Location and distance between coke molecules inside the HBEA zeolite at 30bar and 623K obtained by molecular simulation and pulsed EPR DEER experiment. Display omitted
► HBEA (11) deactivates by ...coking during ethanol transformation. ► Coke molecules were characterized by several spectroscopic techniques. ► The coke was classified into 17 families following their unsaturation numbers. ► Coke molecules inside the pore are mainly hexamethylbenzene or alkyl-pyrenes.
HBEA (11) zeolite was deactivated rapidly by coking during the ethanol transformation into hydrocarbons, at 623K and 30bar. The nature of carbonaceous deposit was studied, after zeolite dissolution by hydrofluoric acid both by gas chromatography coupled with mass spectrometry and by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS). The coke on the external zeolite surface was characterized directly on the spent catalyst by laser desorption/ionization time of flight mass spectrometry (LDI-TOF MS). The coke species were identified and classified into 17 families following their unsaturation number (4–23). The coke was polyaromatic, and it was very alkylated (methyl, ethyl, and propyl groups). It is compounded of alkylbenzenes, mainly hexamethylbenzene (HMB), and alkyl-pyrenes, located within the zeolite pore, which could migrate to the external zeolite surface and grow into polyaromatic compounds constituted up to eight aromatic rings and up to 40 carbon atoms. Pulsed electron paramagnetic resonance spectroscopy measurements and molecular simulation by using Cerius2 software have shown that HMBs are distant of 0.6nm inside the zeolite pores.
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
In the general context of the development of the use of agricultural products for non-food applications, and particularly in the field of the glycerol valorisation (co-product of the triglycerides ...hydrolysis or methanolysis process), the selective etherification of glycerol was studied. The objective of this work is the direct and selective synthesis, from glycerol and without solvent, of polyglycerols having a low polymerisation degree (di- and/or triglycerol), in the presence of solid mesoporous catalysts. The main part of this study consists in the synthesis and the impregnation of mesoporous solids with different basic elements in order to make them active, selective and stable for the target reaction. The catalytic results obtained show that this impregnation method gives important activity, which must be correlated to an important active species incorporation. Concerning the selectivity of the modified mesoporous catalysts, the best value to (di-+tri-) glycerol are obtained over solids prepared by caesium impregnation. The re-use of these caesium impregnated catalysts does not affect the selectivity to the (di-+tri-) glycerol fraction. In the presence of lanthanum or magnesium containing catalysts, the glycerol dehydration to acrolein is very significant whereas this unwanted product is not formed when caesium is used as impregnation promoter.
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•Fluoride and alkaline leaching of MFI zeolite generate additional but different porosities.•Such post-synthesis treatments have no effect on the intrinsic microporosity of their ...parent.•The proximity of intra- or inter-crystalline mesopores with active sites, H+, impedes the growth of coke precursors.•Catalyst lifetime depends on the length of diffusional path.
The effects of two different hierarchization procedures (alkaline and fluoride leaching) on the performances of ZSM-5 catalysts in the transformation of methylcyclohexane at 723K are highlighted and discussed in relation to their porosities. The hierarchical catalysts exhibit different porosities; namely, the fluoride treatment leads to a zeolite combining micropores and macropores, while alkaline leaching adds mesopores interconnected with the native micropores. While the initial activities and selectivities of catalysts derived from the three zeolites are very similar in the conversion of methylcyclohexane, the presence of mesopores (alkaline leaching), close to the active sites, greatly improves the stability of such a hierarchical catalyst by favoring the desorption of products. This behavior is similar to a reduction in zeolite crystal size. This increased stability is not due to a decrease of the coke toxicity, but rather to an inhibition of the growth of coke precursors, in turn related to the shorter diffusion paths of reactants and products. Two types of coke are present on the meso-/micro-porous zeolite: (i) a “light coke” composed of alkylbenzenes strongly adsorbed on Lewis acid sites and silanols, (ii) a “heavy coke” (alkylphenanthrenes and alkylpyrenes) trapped at the intersection of the zeolite channels. While the light coke has no impact on the catalyst stability, the heavy coke poisons active sites, most probably remote from the mesopores.
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