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Mesoporous mordenites obtained by desilication: Mechanistic considerations and evaluation in catalytic oligomerization of pentene
Accepted Manuscript Mesoporous mordenites obtained by desilication: mechanistic considerations and evaluation in catalytic oligomerization of pentene Chloé Bertrand-Drira, Xiao-wei Cheng, Thomas Cacciaguerra, Philippe Trens, Georgian Melinte, Ovidiu Ersen, Delphine Minoux, Annie Finiels, François Fajula, Corine Gerardin PII:
S1387-1811(15)00090-6
DOI:
10.1016/j.micromeso.2015.02.015
Reference:
MICMAT 6989
To appear in:
Microporous and Mesoporous Materials
Received Date: 8 December 2014 Revised Date:
3 February 2015
Accepted Date: 5 February 2015
Please cite this article as: C. Bertrand-Drira, X.-w. Cheng, T. Cacciaguerra, P. Trens, G. Melinte, O. Ersen, D. Minoux, A. Finiels, F. Fajula, C. Gerardin, Mesoporous mordenites obtained by desilication: mechanistic considerations and evaluation in catalytic oligomerization of pentene, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.02.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Mesoporous mordenites obtained by desilication: mechanistic considerations
Chloé Bertrand-Drira1, Xiao-wei Cheng1, Thomas Cacciaguerra1, Philippe Trens1, Georgian Melinte2, Ovidiu Ersen2, Delphine Minoux3, Annie Finiels1, François Fajula1, Corine Gerardin1* Institut Charles Gerhardt, UMR 5253 UM2-CNRS-ENSCM-UM1- Matériaux Avancés pour la
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Catalyse et la Santé, ENSCM 8 rue de l’Ecole Normale, 34095 Montpellier Cedex , France Institut de Physique et Chimie des Matériaux, UMR 7504 CNRS, Strasbourg, France
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Total Research & Technology Feluy, Belgium
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Mesoporous mordenite
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Mesoporous mordenites obtained by desilication: mechanistic considerations and evaluation in catalytic oligomerization of pentene
Chloé Bertrand-Drira1, Xiao-wei Cheng1, Thomas Cacciaguerra1, Philippe Trens1, Georgian
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Melinte2, Ovidiu Ersen2, Delphine Minoux3, Annie Finiels1, François Fajula1, Corine Gerardin1* Institut Charles Gerhardt, UMR 5253 UM2-CNRS-ENSCM-UM1- Matériaux Avancés pour la
Catalyse et la Santé, ENSCM 8 rue de l’Ecole Normale, 34095 Montpellier Cedex, France Institut de Physique et Chimie des Matériaux, UMR 7504 CNRS, Strasbourg, France
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Total Research & Technology Feluy, Belgium
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Corresponding author: [email protected]
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Keywords: zeolite, mesoporous, desilication, hierarchically porous, catalysis Abstract
Desilication of mordenite crystals previously enriched in silica by dealumination has been
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investigated in order to better understand the process of mesopore formation. Pentene oligomerization in the liquid phase in the presence of an alkane solvent has been used to evaluate the catalytic activity of the desilicated zeolite. Relevant physico-chemical characteristics such as texture, porosity, acidity and composition of zeolite samples
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desilicated at 80°C in NaOH solutions at different base concentrations were analyzed quantitatively. The generation of mesoporosity is due to the hydrolysis and dissolution of the
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surface of the crystals, probably initiated by the formation of etch pits at low coordination atoms, followed by surface roughening, formation and growth of internal mesopores, fragmentation and, ultimately, by a complete dissolution of the crystal. Such a process which is similar to the one involved in the chemical weathering of rocks, results in a severe loss of crystalline material and of the strong acidity associated to it. The oligomerization catalyst prepared from desilicated mesoporous mordenite proved highly stable and selective for the production of C15-C30 oligomers from pentene.
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ACCEPTED MANUSCRIPT 1. Introduction Zeolites are among the most widely used heterogeneous catalysts in oil refining and petrochemistry1,2 due to unique properties like high surface area, strong acidity, shape selectivity and high thermal and hydrothermal stability. Because the active surface in zeolites develops inside micropores with sizes around, and mostly below, 1 nm in size,
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several strategies have been developed in order to improve mass transport and use more efficiently the zeolite crystals.3,4 These include the synthesis of zeolites with larger micropores,5,6 the synthesis of nanosized zeolites,7,8 or the creation of a secondary mesoporous framework inside the zeolite, leading to materials with multimodal porosity,
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usually denominated hierarchical zeolites. The practical implementation of the first options is limited because the nanosized zeolite crystals are not easily separated from the reaction
pores are complex and expensive.9
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mixture and the templates generally involved in the synthesis of zeolites with extra large
Several methods for the preparation of hierarchically porous materials were reported in the literature. They were recently reviewed by Chal et al.10 These methods can be divided into two categories: the destructive and the constructive ones. The destructive methods involve
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demetallation post-treatments such as dealumination2 and/or desilication.3,11 They consist in the partial dissolution of the zeolite to selectively remove aluminium or silicon, respectively, from the zeolite framework, generating mesopores into zeolite crystals. In order to better control the size and the distribution of the mesopores, constructive methods have been
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developed, such as the use of hard sacrificial templates, the introduction of zeolite-like cristallinity and microporosity into the amorphous walls of mesoporous materials,12 the use
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of dual templating to directly generate micro and mesopores,13,14 the assembly of zeolite nanocrystals,15 or post-synthesis treatments such as zeolite recrystallization which most often uses surfactants as mesoporogens.16,17 The aim of this work was, on one hand, to get a better insight into the mechanism of desilication of mordenite, a zeolite widely used in catalysis and, on the other hand, to evaluate the effect of the generation of a secondary mesoporous network on the catalyst activity for the oligomerization of n-pentene. Oligomerization of C5-C6 olefins from light cracking naphta constitutes an option for increasing middle distillate production18 or reducing Ried vapor pressure of straight-run gasoline19. Olefin oligomerization over solid 2
ACCEPTED MANUSCRIPT acid catalysts, and particularly zeolites, has been extensively investigated since the 70's and comprehensively reviewed.20 Provided the reaction is run in the liquid phase in the presence of an alkane solvent, solid acid catalysts demonstrate high activity and reasonable resistance towards deactivation.21 Moreover, it has been suggested that with zeolite catalysts the oligomerization process occurs essentially on the mesopores and on the external surface of
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the crystals.22 The pentene oligomerization reaction therefore constitutes an excellent test bed in the frame of this study.
In order to reach our goals, a silica-rich sample of mordenite was used as starting material. After alkaline treatments at different concentrations of sodium hydroxide at 80°C, the
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resulting micro-mesoporous materials were characterized in details with a special focus on composition, acidity, morphology and texture and their quantitative variations, with the aim
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to discuss the main features involved in the process of mesopore generation. A representative sample of desilicated mesoporous mordenite was then used as a catalyst for the oligomerization of n-pentene at 50 barg in the presence of heptane as a solvent.
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2. Experimental section 2. 1. Materials and Treatments. Parent material
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The starting material used was a mordenite zeolite from Zeolyst International denoted as CBV90A. This mordenite was under the H-form with a nominal Si/Al ratio of 48. This
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mordenite with high Si/Al ratio was prepared by the supplier by applying a dealumination treatment to remove part of the framework aluminium. Desilication of mordenite
The alkaline treatments were carried out using sodium hydroxide (NaOH, Sigma) as the desilicating agent. The experimental procedure was the following: 2 g of CBV90A mordenite were stirred in 50 mL of NaOH aqueous solution. The NaOH concentration range was 0.2 – 0.4 M and the treatment was carried out at 80°C during 30 min. After the treatment, the solid was collected by centrifugation, washed with distilled water until pH 7 and dried overnight at 80°C. The alkaline modified solid was then subjected twice to an ion exchange in a 0.2M ammonium 3
ACCEPTED MANUSCRIPT nitrate (NH4NO3, Sigma) solution for 2 h. After washing with distilled water and drying, the product was calcined in static air at 550°C for 8 h. The desilicated samples were named according to the expression: D-[NaOH] (M), for example D-0.2 for desilication in 0.2M NaOH.
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2. 2. Characterization methods. The structural characterization of the parent and modified samples was done by recording powder X-Ray Diffraction (XRD) patterns on a Bruker Lynx Eye diffractometer with BraggBrentano geometry and CuKα radiation (λ=0.15406 nm) as incident beam. Data were
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recorded by continuous scanning in the range 4-50°2θ for studying the crystalline zeolite structure and in the range of 0.5-6°2θ for the mesoporous structure, with an angular step
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size of 0.0197 °2θ and a counting time of 0.2 second per step.
The textural characterization of the solids was done by recording N2 sorption isotherms at 196°C on a Micromeritics TriStar 3000. Prior to the isotherms acquisition, the samples were degassed under vacuum at 250°C for 7 h. The Brunauer-Emmet-Teller (BET) method23 was applied to calculate the specific surface area of the materials. External surface area was calculated using the αs method.24 The micropore volumes (D<2 nm, Vmicro) in the
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hierarchically porous zeolites were determined by using the αs method (far more appropriate than the t-plot method25, 26); the total pore volume (Vtotal) was obtained from the nitrogen amount adsorbed at the relative pressure of about P/P0 = 0.95. The mesopore
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volume (D> 2nm, Vmeso) was set as the difference between Vtotal and Vmicro. The pore size distributions (PSD) were evaluated from the adsorption isotherms by the Density Functional
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Theory method.27
The global Si/Al ratio was determined by inductive coupled plasma mass spectroscopy (ICPMS) and by energy dispersive X-ray (EDX) analysis using a FEI Quanta 200F microscope at 15 kV.
The morphology of the crystals was observed by scanning electron microscopy (SEM) using a FEG Hitachi S-4800 microscope between 0.5 and 30 kV. The electron tomography experiments were carried out on a JEOL 2100F transmission electron microscope with a field emission gun operating at 200 kV, equipped with a probe corrector and a GATAN Tridiem energy filter. Prior to observation, the zeolite crystals were dispersed in deionized water and then sonicated for several minutes. Up to 5 droplets were 4
ACCEPTED MANUSCRIPT further deposited onto a copper grid covered by a holey carbon membrane rendered hydrophobic by H2/Ar plasma cleaning. Tomography series were acquired by tilting the specimen over a range of ±60°, with an image recorded every 2°. The images were aligned by using the cross-correlation algorithm implemented in the IMod software. The reconstructions have been computed using 10 iterations within the algorithms based on
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algebric reconstruction techniques implemented in the TOMOJ software. The visualization and quantitative analysis of the final volumes have been done by using the ImageJ software. The acidic properties of the samples were studied by Temperature-Programmed Desorption of ammonia (NH3-TPD) using a home-made device equipped with a conductivity cell. The
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samples were pre-treated at 550°C under air flow (30 ml/min) for 8 h. After returning down to 100°C the samples were swept by a N2 flow (30 ml/min) during 10 min. Then a pure flow
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of NH3 at 30 ml/min was adsorbed at 100°C for 15 min. In order to remove NH3 weakly adsorbed on the zeolite, a N2 flow (30 ml/min) was passed through the reactor for 4 h. Finally, desorption of NH3 was monitored in the temperature range of 100-550°C using a heating rate of 10 °C/min. 27
Al magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were
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performed at 104.27 MHz at a spinning speed of 20000 tr.min-1 using a Varian VNRMS 400 spectrometer with 3.2 mm rotors.
2.3 Catalytic evaluation
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The mordenite powders (parent and desilicated) were pressed into wafers, crushed and sieved to obtain particles with diameters of 150-250 µm. Catalytic reactions were conducted
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in a tubular fixed-bed down-flow reactor (6 mm internal diameter) loaded with 1g of catalyst. The catalyst was supported by a porous disk (60µm) and the dead volume was filled with quartz particles of 200-400 µm in size. The catalyst temperature was monitored with a thermocouple placed inside the bed. The catalyst, previously activated in flowing air (60 mL/min at 550°C for 8h) was loaded into the reactor and dehydrated at 180°C for three hours in flowing air. Pure n-heptane was then fed to the system using a HPLC pump (Gilson) until the operating pressure (50 barg) was obtained. The n-heptane flow was then shifted to the reagent feedstock, consisting of a 50/50 mixture of pent-1-ene and n-heptane (both
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ACCEPTED MANUSCRIPT from Sigma-Aldrich, 99% purity, WHSV: 0.5-2 h-1). Reactor pressure was regulated using an Equilibar back-pressure regulator. Reaction products were analyzed on line by gas chromatography using a DB 2887 capillary column (100% dimethylpolysiloxane, L=10m, ID = 0.53 mm, film thickness = 3 µm). The families of oligomers, nC5=, with n = 2 to 6 , were identified by GC-MS analysis and lumped
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according to the value of n. Total conversion was calculated from the amount of unconverted linear pentenes (pent-1-ene and c+t-pent-2-enes are readily equilibrated by double bond isomerization under the reaction conditions). Carbon balances higher than 98% were obtained in all runs.The degree of skeletal branching of the dimer fraction (n=2) was
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determined by identifying the isomers contained in the mixture of decenes after hydrogenation.
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3. Results Desilication of mordenite
Series of desilication treatments for 30 min at 80°C with NaOH solutions at different concentrations were performed on the zeolite mordenite CBV90A in order to generate mesoporosity in the crystals. The main characteristics, texture, composition and total acidity
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of the parent and desilicated samples are presented in Table 1.
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Table 1.
Desilication of zeolites with intermediate Al contents involves the selective extraction of
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silicon atoms resulting in a decrease of the Si/Al ratio and a significant loss of material at high base concentrations.
Wide-angle XRD patterns (Fig S1) of the parent mordenite CBV90A and of the desilicated samples exhibited the typical diffraction peaks of the mordenite framework. The XRD patterns were barely affected by the desilication treatment, except for the sample desilicated in a 0.4M NaOH solution for which a decrease of the intensity of the diffraction peaks and of the signal-to-noise ratio was noticed, suggesting some loss of long range crystallinity. This effect has also been observed by Paixão et al., van Laak et al., and Li et al.2831
Nevertheless, the XRD patterns do not reveal any signature of an amorphous phase.
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ACCEPTED MANUSCRIPT N2 adsorption-desorption isotherms of the parent mordenite CBV90A and treated samples are displayed on Figure 1-A. The curve of the parent mordenite shows the presence of mesopores, with a broad size distribution, formed during the dealumination post-treatment of the parent mordenite. The small step at P/P0 = 0.42 in the desorption branch of the isotherm is typical for a cavitation phenomenon indicating that some of the mesopores are
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connected to the crystal surface via restrictions.
The isotherms of the desilicated samples still present a sharp rise at low relative pressures showing preservation of the microporosity whose amount is barely modified when NaOH concentration increases. Concomitantly, the development of mesopores connected to the
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crystals surface increases with increasing NaOH concentration; nevertheless the cavitation signature remains present, though reduced, in all samples.
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Pore size distributions for the parent zeolite (Figure 1-B) calculated on the adsorption branch reveal a narrow size distribution of mesopores centered at 4 nm and a broader contribution of larger pores centered at ca 10 and 40 nm. The mesopores in the desilicated samples exhibit a broad distribution of sizes spreading from 5 to 20 nm, with a maximum at around
Figure 1
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6-8 nm, and a minor contribution of larger mesopores centered at 40-50 nm.
A better view of the textural changes induced upon desilication is gained from the analysis of
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SEM and TEM micrographs of the parent and modified samples (Figure 2, Figures S2 and S3). The parent mordenite CBV90A is constituted of particles with a large distribution of sizes
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(0.3-2 µm) consisting of aggregates of small boulder-like crystals (70-300 nm) with a smooth surface. Examination of the crystals by TEM reveals some textural heterogeneity with zones featuring intracrystalline mesopores with sizes in the range 3-10 nm. Desilication with 0.2M NaOH resulted in some agglomeration of the individual smaller crystals but above all in a severe dissolution of their surface giving rise to a significant surface roughness. At the same time the density and size of the intracrystalline mesopores increased (Figs S2, S3). At even higher NaOH concentrations (0.3 and 0.4 M) the crystal dissolution process was more pronounced. The sample was formed of aggregates (0.2-2 µm) of highly (meso)porous loosely bonded nanoparticles, some of them of very small sizes (5-15 nm) (Figure 2). As a 7
ACCEPTED MANUSCRIPT consequence the external surface area of the crystals increased (Table 1), reaching 100 m²/g for the highly desilicated D-0.4 material. The 3D-TEM cross-section images of the reconstructed volumes of sample D-0.3 are shown in Figure 3. The two images correspond to a numerical cross section in the XY direction of about 1 nm and they are separated by about 60 nm in the Z direction. The 3D TEM
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observation clearly reveals the presence of an intracrystalline 3-dimensional mesopore network, spread over the whole crystal volume, consisting of cavities and channels with sizes in the range 3-15 nm, well connected, at least for the largest ones, to the external surface.
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Figure 2
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Figure 3
The generation of mesopores in high silica-to-alumina zeolites upon treatment by alkaline solution is the result of the dissolution of silica, or silica-rich crystalline domains. As a consequence both Si/Al and Vmicro/Vtotal (VTotal corresponding here to the volume of adsorbed
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nitrogen at P/P0 = 0.95) ratios of the desilicated samples decrease when the severity of the alkaline treatment increases (Table 1). The significant increase of the amount of external surface area, and above all the evolution of the texture of the materials evidenced by electron microscopy, indicate however that the additional (meso)porous volume generated
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is not fully associated with intracrystalline mesopores. In Figure 4 we have represented the linear relationship existing between the micro and mesoporous volumes of the four
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mordenite samples. On a per weight basis, increasing the severity of the alkaline treatment leads to an increase in mesoporous volume of the analyzed material without apparent modification of the microporous volume. This trend is similar to the observation reported by van Laak et al. in the case of the desilication of mordenite crystals under mild alkaline conditions.31 Such a representation, which is the one generally reported in literature, does not give however a realistic picture of the phenomena associated with the desilication process. Indeed, as shown in Figure 4, when taking into consideration the actual yield of the reaction, ie if the different porous volumes are normalized to the initial weight of parent zeolite engaged, under the conditions of our study, the amount of mesopore volume is 8
ACCEPTED MANUSCRIPT barely affected by the severity of the desilication treatment. Such a phenomenon can be readily explained at the light of the desilication efficiency descriptor introduced by Verboekend et al.32 which relates the mesopore area and volume generated to the mass of zeolite dissolved and the size of the individual crystals. As the alkaline treatment results not only in the generation of intracrystalline mesopores but also in the roughening of the zeolite
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surface, increasing the severity of the treatment leads to a fragmentation of the crystals. When the size of the crystal fragments approaches that of the intracrystalline mesopores, the generation of additional mesoporosity becomes less (or no longer) efficient as the main process involved is the dissolution of the nanosized zeolite crystals. In the present study it is
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clear that the alkaline treatment performed with a 0.2M solution of sodium hydroxide leads essentially to surface roughening besides minimal creation of internal mesoporosity. At
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higher base concentrations crystal fragmentation and dissolution are the major processes taking place, no formation of additional intracrystalline mesopores can be detected.
Figure 4
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Acidity
The NH3-TPD profile of the parent mordenite CBV90A (Figure 5) shows two broad peaks, a small one at low temperature (at about 210°C) associated with weak acid sites and a more intense one (80% of the total acidity) at high temperature (around 510°C) associated with
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strong acid sites. The TPD profiles of the desilicated samples (Figure 5-A, Table 1) show that the total number of acid sites per unit of weight increases as NaOH concentration increases
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due essentially to an increase of the number of weak acid sites. This increase is accompanied by a slight shift of the desorption temperature to higher values (from 210 to 310°C).
Figure 5
The increase of the total number of acid sites per unit of weight is consistent with the decrease in Si/Al ratio, as the material becomes richer in Al, a phenomenon also reported by Groen and co-workers33 and X. Wei and P.G. Smirniotis34, for example. However, the increase in the number of acid sites is not strictly proportional to the increase in aluminium content of the sample as seen in Table 2. While in the starting material 90% of the Al atoms 9
ACCEPTED MANUSCRIPT of the sample generate an acid site, nearly half of the Al atoms do not contribute to acidity on the highly desilicated sample.
Table 2. Moreover, if the TPD profiles are normalized to the initial amount of zeolite (Figure 5-B) a
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somewhat different picture is obtained as it clearly appears that, in fact, the number of weak acid sites is barely maintained constant after desilication while the number of strong acid sites is dramatically decreased. Desilication results therefore in the removal of strong acid sites through the dissolution of the zeolitic framework and the disappearance of
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tetrahedrally coordinated aluminium. Actually, in the more severely desilicated sample a flat distribution of acid strengths is observed indicative of a highly faulted zeolite network.
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This conclusion is supported by the 27Al NMR spectra of the samples (Figure 6). The parent material exhibits a single signal at 54 ppm indicating that most of the Al is located in the framework in tetrahedral sites and prone to generate strong acid sites in agreement with the quantitative titration using ammonia. In the desilicated samples a new signal characteristic of aluminum atoms in octahedral sites not covalently bonded to the zeolite lattice appears at
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0 ppm and the linewidth of the peak at 54 ppm broadens, indicating a less symmetrical environment of the lattice tetrahedrally bonded Al atoms.
Catalytic activity
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The catalytic behavior of two samples, namely the parent mordenite CBV90A and the desilicated D-0.2 sample, has been investigated in the oligomerization of pent-1-ene at
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180°C under 50 barg of pressure and a WHSH of 1 h-1. These reaction conditions have been selected on the bases of a series of preliminary experiments performed at varying temperatures and space velocities (Figures S5 and S6). Under the conditions of the test the two catalysts were very active, leading to initial conversions above 95%. The reaction products consisted solely in mixtures of nC5= oligomers, with n comprised between 2 and 6. No evidence for the occurrence of cracking reactions was obtained (Figure 7). Detailed analysis of the composition of the hydrogenated dimer fraction (C10) on both catalysts at different times on stream was achieved in order to determine the degree of branching of the C10 oligomers. This revealed the absence of isomerization reactions leading to a change in 10
ACCEPTED MANUSCRIPT the branching of the hydrocarbon chain (Type B rearrangements) under the conditions of the reaction. The obtained products consisted of 60 ± 2% monobranched hydrocarbons (>90% methyl-nonenes and ethyl-octenes), 38 ± 2% dibranched hydrocarbons ( 80% dimethyloctenes, 20% diethyl-hexenes plus methyl-ethyl heptenes) with only marginal amounts ( < 2%) of linear decenes. Such a product distribution is well accounted for by classical
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alkylcarbenium ion chemsitry involving addition of the secondary carbenium ions generated from pent-1-ene and pent-2-enes to the parent olefins followed by c:t and double bond migration and eventually alkyl shifts (Type A rearrangement) and ultimately, deprotonation35. As regards the catalyst stability and selectivity of the oligomer products,
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the effect of the generation of a secondary network of mesopores by desilication, which generates external surface area and improves mass transfer inside zeolite crystals is clearly
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apparent from Figure 7 and table 3. Stability of the activity with time on stream was significantly improved and the oligomers distribution was shifted towards heavier (C15-C30) products in the D-0.2 sample. Both effects result from enhanced mass transfer and better desorption of the heavier oligomers, and of the poison precursors, from the zeolite crystals featuring a high density of mesopores22,36 due to shorter path lengths.37,38 The catalyst
4. Discussion
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demonstrated moreover high stability with regards repeated reaction/regeneration cycles.
In this study the desilication of mordenite crystals, previously enriched in silica by
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dealumination, at increasing severity of the alkaline treatment has been followed using different characterization techniques and analyzed quantitatively in order to reach a clear
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and realistic description of the phenomena involved in the process of mesopore formation. On the basis of our observations it can be concluded that the generation of mesoporosity starts by the hydrolysis and dissolution of silicate species near the surface of the crystals as already suggested by van Laak et al.31 This process is more probably initiated at defect sites, and results in a progressive surface roughening and creation of some intracrystalline mesoporosity, followed by fragmentation and, ultimately, in a complete dissolution of the crystal. Interestingly, the estimation of the mesopore volume created relative to the weight of the initial zeolite brings to the conclusion that most of the material removed from the crystal by dissolution does not contribute to the formation of intracrystalline mesoporosity. As a consequence of this dissolution process, the development of mesoporosity in the 11
ACCEPTED MANUSCRIPT mordenite crystals is accompanied by a severe loss of zeolitic crystalline material and of the strong acidity associated to it. The observed dissolution figures, namely the surface roughening and the shapes of the intracrystalline mesopores, cannot be interpreted on the bases of the structural features of the mordenite framework but more probably by considering the composition of the zeolite
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and, more precisely the distribution of the tetrahedral aluminium sites in the lattice. Such a statement is in line with the well known influence of structural aluminium in controlling zeolite dissolution.
Regarding crystal morphology changes reported here for mordenite, our observations
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closely resemble those made by Verboekend et al.32 in the case of the desilication of nanometric rod-like ZSM-22 and by Bonilla et al.33 for the desilication of aggregates of thin
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platelets of ferrierite but contrast with those reported in the desilication of large mordenite33,34 and ZSM-540 crystals in NaOH solution. In the latter cases the alkaline attack allowed generating high mesoporosity and external surface while preserving the original morphology of the crystals.
Similar contrasting behaviors have been pointed out and tentatively rationalized by correlating the amount of mesopores or external surface generated and the amount of
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zeolite dissolved to the initial size of the crystals, as said above.32 Actually, the occurrence of non uniform dissolution patterns is a rather common phenomenon observed during the chemical weathering of rocks, including silicate minerals, and can be extended to the
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dissolution of zeolites as we reported some years ago41 in a study dealing with the crystallization and subsequent dissolution of mazzite in various media. Zeolite crystals in a
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highly undersaturated medium dissolve via the formation of etch pits on all exposed surfaces. When this occurs, etching takes place preferentially at dislocations or defects that intercept the crystal surface. Low index, flat, crystal faces are less affected because of their lower density of defects resulting from their lower growth rates. Dissolution by etching occurs therefore essentially on high index faces, at corners or edges, on atoms with lower coordination. The dissolution rate of the various faces determines crystal habit, as it is the case for crystal growth. It is therefore clear that crystal morphology, and more generally speaking the history of the parent crystal submited to desilication by alkaline attack, can play
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ACCEPTED MANUSCRIPT a role as important as composition and structure in determining the outcome of the desilication process. As regards the catalytic behavior of the desilicated mordenite, the generation of intracrystalline mesopores and the decrease of the size of the crystals result in a significant improvement of the stability and of the activity of the catalyst in the oligomerization of
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linear pentenes and a shift of the product distribution towards heavier oligomers, which is well in line with an easier desorption of the reaction intermediates from the acidic sites.
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5. Conclusion
Quantitative analysis of the textural, composition and acidity changes occurring during the
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desilication of zeolite crystals, previously enriched in silicon by dealumination, using NaOH solutions with increasing base concentrations allowed us to draw a consistent scheme of the mechanism of mesopore formation. The generation of mesoporosity is due to the hydrolysis and dissolution of the surface of the crystals, probably initiated by the formation of etch pits at low coordination atoms, leading to surface roughening, formation and growth of internal mesopores followed by fragmentation and ultimately, by a complete dissolution of the
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crystal. Such a process results in a severe loss of crystalline material and of the strong acidity associated to it. As a consequence of an easier intracrystalline mass transport and a decrease in particle size, the catalytic activity of the desilicated mordenite in the
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oligomerization of pentenes was significantly improved.
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Acknowledgements
TOTAL Research and Technology Center of Feluy (Belgium) is acknowledged for funding the present study. C. Bertrand-Drira is especially thankful to TOTAL for her grant. References
[1] W. Vermeiren, J.P. Gilson, Topics in Catalysis 52 (2009) 1131-1161. [2] S. van Donk, A.H. Janssen, J.H. Bitter, K.P. de Jong, Catal. Rev. 45 (2003) 297-319. [3] J. Perez-Ramirez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen, Chem. Soc. Rev. 37 (2008) 2530-2542. [4] C.H. Christensen, K. Johannsen, E. Tornqvist, I. Schmidt, H. Topsoe, C.H. Christensen, Catal. Today 128 (2007) 117-122. 13
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[5] C.C. Freyhardt, M. Tsapatsis, R.F. Lobo, K.J. Balkus, E. Davis, Nature 381(1996) 295-298. [6] A. Corma, M.J. Díaz-Cabañas, J.L. Jordá, C. Martínez, M. Moliner, Nature 443 (2006) 842845. [7] M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, R. Ryoo, Nature 461 (2009) 246-249. [8] L. Tosheva, V.P. Valtchev, Chem. Mater. 17 (2005) 2494-2513. [9] F.N. Gu, F. Wei, J.Y. Yang, N. Lin, W.G. Lin, Y. Wang, J.H. Zhu, Chem. Mater. 22 (2010) 2442-2450. [10] R. Chal, C. Gérardin, M. Bulut, S. van Donk, Chem. Cat. Chem. 3 (2011) 67-81. [11] Y.S. Tao, H. Kanoh, L. Abrams, K. Kaneko, Chem. Rev. 106 (2006) 896-910. [12] K.R. Kloetstra, H. Van Bekkum, J.C. Jansen, Chem. Commun. (Camb) (1997) 2281-2282. [13] K.R. Kloetstra, H.W. Zandbergen, Microporous Materials 6 (1996) 287-293. [14] B.T. Holland, L. Abrams, J. Amer. Chem. Soc. 121 (1999) 4308-4309. [15] Y. Liu, W. Zhang, T.J. Pinnavaia, J. Amer. Chem. Soc.122 (2000) 8791-8792. [16] I.I. Ivanova, E.E. Knyazeva, Chem. Soc. Rev. 42(9) (2013) 3671-3688. [17] R. Chal, T. Cacciaguerra, S. van Donk and C. Gérardin, Chem. Commun. 46 (2010) 7840-7842 [18] G. Bellussi, F. Mizia, V. Calemna, P. Pollesel, R. Millini, Microp. Mesop. Mater. 164 (2012) 127-134. [19] R. Schmidt, M.B. Welch, B.B. Randolph, Energy & Fuels 22 (2008) 1148-1155. [20] Ch. Marcilly, Catalyse acido-basique, Application au raffinage, Vol.2, Editions Technip, Paris, 2003, pp 475-506 [21] J.P.G. Pater, P.A. Jacobs, J.A. Martens, J. Catal. 179 (1998) 477-482. [22] J.P.G. Pater, P.A. Jacobs, J.A. Martens, J. Catal. 184 (1999) 262-267.
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[23] S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc. 60 (1938) 225-227. [24] M.R. Bhambhani, P.A. Cutting, K.S.W. Sing, D.H. Turk, J. Coll. Interf. Sci. 38 (1972) 109117. [25] M. Robitzer, F. Di Renzo, F. Quignard, Microp. Mesop. Mater. 140 (2011) 9-16 [26] A. Galarneau, F. Villemot, J. Rodriguez, F. Fajula, B. Coasne, Langmuir 30 (2014) 1326613274 [27] A.V. Neimark, P.I. Ravikovitch, Microp. Mesop. Mater. 44-45 (2001) 697-707. [28] V. Paixão, A.P. Carvalho, J. Rocha, A. Fernandes, A. Martins, Microp. Mesop. Mater. 131 (2010) 350-357. [29] V. Paixão, R. Monteiro, M. Andrade, A. Fernandes, J. Rocha, A.P. Carvalho, A. Martins, Appl. Catal. A-Gen 402 (2011) 59-68. [30] X. Li, R. Prins, J.A. van Bokhoven, J. Catal. 262 (2009) 257-265. [31] A.N.C. van Laak, S.L. Sagala, J. Zecevic, H.Friedrich, P.E.de Jongh, K.P. de Jong, J. Catal. 276 (2010) 170-180 [32] D. Verboekend, A.M. Chabaneix, K. Thomas, J.-P. Gilson, J. Pérez-Ramírez, CrystEngComm 13 (2011) 3408-3416. 14
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[33] J.C. Groen, T. Sano, J.A. Moulijn, J. Pérez-Ramirez, J. Catal. 251 (2007) 21-27. [34] X. Wei, P.G. Smirniotis, Microporous and Mesoporous Materials 97 (2006) 97-106. [35] G.A. Olah, A. Molnar,. Hydrocarbon Chemistry, 2nd ed.; Wiley-Interscience: New York, 2003. [36] A. Corma, C. Martinez, E. Doskocil, J. Catal., 300 (2013) 183- 196. [37] S. van Donk, A. Broersma, OLJ. Giizeman, J.A. von Bohkoven, J.H. Bitter, K.P. de Jong, J. Catal. 204 (2001) 272-280 [38] M. Tromp, J.A. von Bohkoven, M.T.Garriga Oostenbrink, J.H. Bitter, K.P. de Jong, D.C. Koningsberger, J. Catal. 190 (2000) 209-214 [39] A. Bonilla, D. Baudouin, J. Pérez-Ramirez, J. Catal. 2065 (2009) 170-180. [40] J.C. Groen, W. Zhu, S. Brower, S.J. Huynink, F. Kapteijn, J.A. Moulijn, J. Pérez-Ramirez, J. Amer. Chem. Soc. 129 (2007) 355-360. [41] F. Di Renzo, F. Fajula, F. Figueras, S. Nicolas, T. Des Courieres, Stud. Surf. Sci. Catal. 49 (1989) 119-132.
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Table 3. Conversion and oligomer selectivity as a function of catalyst and time on stream (Same conditions as in Figure 7) CBV90A
D-0.2 70 h 78 0.37 0.41 0.22
40h 95 0.10 0.29 0.61
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20h 89 0.25 0.43 0.32
75 h 94 0.13 0.32 0.55
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Catalyst Time on stream (h) Pentene conversion (%) C10= C15= C20= - C30=
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Mesoporous mordenites obtained by desilication: mechanistic considerations and evaluation in catalytic oligomerization of pentene
Chloé Bertrand-Drira1, Xiao-wei Cheng1, Thomas Cacciaguerra1, Philippe Trens1, Georgian
1
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Melinte2, Ovidiu Ersen2, Delphine Minoux3, Annie Finiels1, François Fajula1, Corine Gerardin1* Institut Charles Gerhardt, UMR 5253 UM2-CNRS-ENSCM-UM1- Matériaux Avancés pour la
Catalyse et la Santé, ENSCM 8 rue de l’Ecole Normale, 34095 Montpellier Cedex, France Institut de Physique et Chimie des Matériaux, UMR 7504 CNRS, Strasbourg, France
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Total Research & Technology Feluy, Belgium
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Highlights :
Desilication generates surface roughening and inter-and intracystalline mesopores. Material dissolution does not necessarily lead to intracrystalline mesopores.
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Desilication process resembles chemical weathering of rocks.
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Catalytic pentene oligomerization is largely improved with desilicated mordenites.
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Figure captions
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Figure 1: A. N2 adsorption-desorption isotherms of the parent mordenite CBV90A and desilicated samples, B. Pore size distributions. Figure 2: SEM (left) and TEM (right) images of the parent mordenite CBV90A and of the D- 0.3 desilicated sample
correspond to slices separated by about 60 nm
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Figure 3. Cross-section images of 3D-TEM reconstructed volumes of D-0.3 sample. The two images
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Figure 4. Two presentations of the relationship between the micro and mesoporous volumes in desilicated mordenites (per gram of analyzed material (diamonds), and per gram of initial zeolite (triangles))
Figure 5. NH3-TPD spectra of the parent mordenite CBV90A and of the desilicated samples. A: per gram of analysed material, B: per gram of initial zeolite
Figure 6. 27Al MAS NMR spectra of parent mordenite and D- 0.2 and D- 0.4 desilicated samples
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Figure 7. Oligomerization of pentene over the parent CBV90 (diamonds) and desilicated D-0.2 (squares) catalysts at 180°C, 50 barg, WHSV = 1h-1. Inset: typical chromatogram with oligomer distribution obtained over D-0.2 catalyst.
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Supplementary information : Figure Captions Figure S1: XRD patterns of the parent and desilicated samples of mordenite
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Figure S2: Scanning Electron Micrographs of the parent and desilicated samples of mordenite Figure S3: Transmission Electron Micrographs of the parent and desilicated samples of mordenite Figure S4. A. 3D surface model of the selected D-0.3 zeolite grain; B. Longitudinal (XY) slice through the 3D model of the selected D-0.3 zeolite grain. Figure S5: Catalytic activity of catalyst D-0.2 for pentene oligomerization at different temperatures Figure S6: Influence of space velocity on the stability of the activity (catalyst D-0.2)
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Table 1. Porosity and acidic properties of the parent and desilicated mordenites at different NaOH concentrations and their corresponding Si/Al and yields of the desilication treatments Vmicro c (ml g-1)
Vmeso d (ml g-1)
CBV90A
588
50
0.20
0.08
D-0.2
543
77
0.18
0.17
D-0.3
591
89
0.18
D-0.4
542
103
0.17
Sample
Vtotal e
Yield
(ml g-1)
Si/Al f (at/at)
NH3 TPD (mmol g-1)
(wt%)
0.28
48
0.31
/
0.35
35
0.39
57
0.29
0.47
24
0.45
37
0.37
0.54
16
0.54
21
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Sextb (m² g-1)
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SBET a (m² g-1)
BET method, b αs method, c Volumes determined using αs method , d difference between Vtotal and Vmicro, e Volume adsorbed at P/P0=0.95 on the N2 adsorption branch, f EDX.
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Table 2. Total acidity and amount of weak and strong acid sites of the mordenite parent and desilicated samples
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Si/Al
Sample
Al/(Al+Si)
mmol Al/g
Total acidity mmol.g-1
Weak acid sites mmol.g-1
Strong acid sites mmol.g-1
(at/at)
CBV90A
48
0.0204
0.34
0.31
0.06 (20%)
0.25 (80%)
D-0.2
35
0.0278
0.46
0.39
0.14 (35%)
0.25 (65%)
D-0.3
24
0.0407
0.68
0.45
0.18 (39%)
0.27 (61%)
D-0.4
16
0.0584
0.98
0.54
0.25 (46%)
0.29 (54%)
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A
D-0.4 400
D-0.3 D-0.2 CBV90A
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Vads / ml.g-1
350
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450
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Figure 1A
300 250 200 150
50 0
0.2
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100
0.4
0.6 P/P0 / -
0.8
1
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0.07
B
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Figure 1B
D-0.4 D-0.3
0.06
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PSD / ml.g-1.nm -1
D-0.2
0.05 0.04 0.03
0.01 0
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0.02
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1
10 Pore diameter / nm
CBV90A
100
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Figure 2
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Figure 3
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Figure 4 0.25
0.2
CBV90A
D-0.2
0.1
0 0
0.05
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D-0.4
0.1
0.15
0.2
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Vmeso / mL.g-1
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D-0.4
Normalization to the initial amount of zeolite
D-0.3 0.05
D-0.3
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0.15
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Vmicro / mL.g-1
D-02
0.3
0.35
0.4
Figure 5A
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D-0.4
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D-0.3 D-0.2
CBV90A
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8 6 4
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Intensity / u.a.
10
2 0
100
200
300 400 500 Temperature / °C
600
700
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Figure 5B
D-0.4 D-0.3
10
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Intensity / u.a.
D-0.2 8
4 2 0 200
300 400 500 Temperature / °C
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100
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600
700
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Figure 6
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Figure 7
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D-0.4
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Intensity / u.a.
D-0.3
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Figure S1
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D-O.2
CBV90A
10
15
20
25 30 35 2 θ / degrees
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40
45
50
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Figure S2
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Figure S3
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Figure S4
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Figure S5
9
150°C 50barg WHSV=1h-1
180°C 50barg WHSV=1h-1
90
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70 60 50
Sélectivités C10 : 0,11 C15 : 0,30 C20+ : 0,59
Sélectivités C10 : 0,26 C15 : 0,36 C20+ : 0,38
150°C 50 barg WHSV = 1 h-1
30 20
180°C 50 barg WHSV = 1 h-1
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kD=0,0002 h-1
kD =0,0098 h-1
10 0 10
20
30
40
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0
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TOS (h)
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Conversion (%)
80
G-D90-0,2Na-0,5 Si/Al=35 Acidité=0,41 mmol/g Vmicro=0,19 ml/g VLméso=0,21ml/g
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100
8
50
60
70
80
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Figure S6 1
2
180°C 50barg WHSV=1h-1
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180°C 50barg WHSV=2h-1
WHSV=1h
100 90
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Sélectivités C10 : 0,19 C15 : 0,49 C20+ : 0,32
70 60
180°C 50 barg WHSV = 1 h-1
50 40
Sélectivités C10 : 0,18 C15 : 0,41 180°C C20+ : 0,41
50 barg WHSV = 1 h-1
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Conversion (%)
80
Sélectivités 180°C C10 : 0,27 50 barg C15 : 0,41 -1 WHSV C20+ : 0,32= 2 h
30 20
kD =0,0005h-1
10
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kD =0,0015h-1 kD =0,0002h-1
0 0
30
Régénération 500°C 8h air
60
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TOS (h)
90
Arrêt chauffage pression
120
S1387-1811(15)00090-6
DOI:
10.1016/j.micromeso.2015.02.015
Reference:
MICMAT 6989
To appear in:
Microporous and Mesoporous Materials
Received Date: 8 December 2014 Revised Date:
3 February 2015
Accepted Date: 5 February 2015
Please cite this article as: C. Bertrand-Drira, X.-w. Cheng, T. Cacciaguerra, P. Trens, G. Melinte, O. Ersen, D. Minoux, A. Finiels, F. Fajula, C. Gerardin, Mesoporous mordenites obtained by desilication: mechanistic considerations and evaluation in catalytic oligomerization of pentene, Microporous and Mesoporous Materials (2015), doi: 10.1016/j.micromeso.2015.02.015. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Mesoporous mordenites obtained by desilication: mechanistic considerations
Chloé Bertrand-Drira1, Xiao-wei Cheng1, Thomas Cacciaguerra1, Philippe Trens1, Georgian Melinte2, Ovidiu Ersen2, Delphine Minoux3, Annie Finiels1, François Fajula1, Corine Gerardin1* Institut Charles Gerhardt, UMR 5253 UM2-CNRS-ENSCM-UM1- Matériaux Avancés pour la
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1
Catalyse et la Santé, ENSCM 8 rue de l’Ecole Normale, 34095 Montpellier Cedex , France Institut de Physique et Chimie des Matériaux, UMR 7504 CNRS, Strasbourg, France
3
Total Research & Technology Feluy, Belgium
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Mesoporous mordenite
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Mesoporous mordenites obtained by desilication: mechanistic considerations and evaluation in catalytic oligomerization of pentene
Chloé Bertrand-Drira1, Xiao-wei Cheng1, Thomas Cacciaguerra1, Philippe Trens1, Georgian
1
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Melinte2, Ovidiu Ersen2, Delphine Minoux3, Annie Finiels1, François Fajula1, Corine Gerardin1* Institut Charles Gerhardt, UMR 5253 UM2-CNRS-ENSCM-UM1- Matériaux Avancés pour la
Catalyse et la Santé, ENSCM 8 rue de l’Ecole Normale, 34095 Montpellier Cedex, France Institut de Physique et Chimie des Matériaux, UMR 7504 CNRS, Strasbourg, France
3
Total Research & Technology Feluy, Belgium
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Corresponding author: [email protected]
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Keywords: zeolite, mesoporous, desilication, hierarchically porous, catalysis Abstract
Desilication of mordenite crystals previously enriched in silica by dealumination has been
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investigated in order to better understand the process of mesopore formation. Pentene oligomerization in the liquid phase in the presence of an alkane solvent has been used to evaluate the catalytic activity of the desilicated zeolite. Relevant physico-chemical characteristics such as texture, porosity, acidity and composition of zeolite samples
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desilicated at 80°C in NaOH solutions at different base concentrations were analyzed quantitatively. The generation of mesoporosity is due to the hydrolysis and dissolution of the
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surface of the crystals, probably initiated by the formation of etch pits at low coordination atoms, followed by surface roughening, formation and growth of internal mesopores, fragmentation and, ultimately, by a complete dissolution of the crystal. Such a process which is similar to the one involved in the chemical weathering of rocks, results in a severe loss of crystalline material and of the strong acidity associated to it. The oligomerization catalyst prepared from desilicated mesoporous mordenite proved highly stable and selective for the production of C15-C30 oligomers from pentene.
1
ACCEPTED MANUSCRIPT 1. Introduction Zeolites are among the most widely used heterogeneous catalysts in oil refining and petrochemistry1,2 due to unique properties like high surface area, strong acidity, shape selectivity and high thermal and hydrothermal stability. Because the active surface in zeolites develops inside micropores with sizes around, and mostly below, 1 nm in size,
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several strategies have been developed in order to improve mass transport and use more efficiently the zeolite crystals.3,4 These include the synthesis of zeolites with larger micropores,5,6 the synthesis of nanosized zeolites,7,8 or the creation of a secondary mesoporous framework inside the zeolite, leading to materials with multimodal porosity,
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usually denominated hierarchical zeolites. The practical implementation of the first options is limited because the nanosized zeolite crystals are not easily separated from the reaction
pores are complex and expensive.9
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mixture and the templates generally involved in the synthesis of zeolites with extra large
Several methods for the preparation of hierarchically porous materials were reported in the literature. They were recently reviewed by Chal et al.10 These methods can be divided into two categories: the destructive and the constructive ones. The destructive methods involve
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demetallation post-treatments such as dealumination2 and/or desilication.3,11 They consist in the partial dissolution of the zeolite to selectively remove aluminium or silicon, respectively, from the zeolite framework, generating mesopores into zeolite crystals. In order to better control the size and the distribution of the mesopores, constructive methods have been
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developed, such as the use of hard sacrificial templates, the introduction of zeolite-like cristallinity and microporosity into the amorphous walls of mesoporous materials,12 the use
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of dual templating to directly generate micro and mesopores,13,14 the assembly of zeolite nanocrystals,15 or post-synthesis treatments such as zeolite recrystallization which most often uses surfactants as mesoporogens.16,17 The aim of this work was, on one hand, to get a better insight into the mechanism of desilication of mordenite, a zeolite widely used in catalysis and, on the other hand, to evaluate the effect of the generation of a secondary mesoporous network on the catalyst activity for the oligomerization of n-pentene. Oligomerization of C5-C6 olefins from light cracking naphta constitutes an option for increasing middle distillate production18 or reducing Ried vapor pressure of straight-run gasoline19. Olefin oligomerization over solid 2
ACCEPTED MANUSCRIPT acid catalysts, and particularly zeolites, has been extensively investigated since the 70's and comprehensively reviewed.20 Provided the reaction is run in the liquid phase in the presence of an alkane solvent, solid acid catalysts demonstrate high activity and reasonable resistance towards deactivation.21 Moreover, it has been suggested that with zeolite catalysts the oligomerization process occurs essentially on the mesopores and on the external surface of
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the crystals.22 The pentene oligomerization reaction therefore constitutes an excellent test bed in the frame of this study.
In order to reach our goals, a silica-rich sample of mordenite was used as starting material. After alkaline treatments at different concentrations of sodium hydroxide at 80°C, the
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resulting micro-mesoporous materials were characterized in details with a special focus on composition, acidity, morphology and texture and their quantitative variations, with the aim
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to discuss the main features involved in the process of mesopore generation. A representative sample of desilicated mesoporous mordenite was then used as a catalyst for the oligomerization of n-pentene at 50 barg in the presence of heptane as a solvent.
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2. Experimental section 2. 1. Materials and Treatments. Parent material
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The starting material used was a mordenite zeolite from Zeolyst International denoted as CBV90A. This mordenite was under the H-form with a nominal Si/Al ratio of 48. This
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mordenite with high Si/Al ratio was prepared by the supplier by applying a dealumination treatment to remove part of the framework aluminium. Desilication of mordenite
The alkaline treatments were carried out using sodium hydroxide (NaOH, Sigma) as the desilicating agent. The experimental procedure was the following: 2 g of CBV90A mordenite were stirred in 50 mL of NaOH aqueous solution. The NaOH concentration range was 0.2 – 0.4 M and the treatment was carried out at 80°C during 30 min. After the treatment, the solid was collected by centrifugation, washed with distilled water until pH 7 and dried overnight at 80°C. The alkaline modified solid was then subjected twice to an ion exchange in a 0.2M ammonium 3
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2. 2. Characterization methods. The structural characterization of the parent and modified samples was done by recording powder X-Ray Diffraction (XRD) patterns on a Bruker Lynx Eye diffractometer with BraggBrentano geometry and CuKα radiation (λ=0.15406 nm) as incident beam. Data were
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recorded by continuous scanning in the range 4-50°2θ for studying the crystalline zeolite structure and in the range of 0.5-6°2θ for the mesoporous structure, with an angular step
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size of 0.0197 °2θ and a counting time of 0.2 second per step.
The textural characterization of the solids was done by recording N2 sorption isotherms at 196°C on a Micromeritics TriStar 3000. Prior to the isotherms acquisition, the samples were degassed under vacuum at 250°C for 7 h. The Brunauer-Emmet-Teller (BET) method23 was applied to calculate the specific surface area of the materials. External surface area was calculated using the αs method.24 The micropore volumes (D<2 nm, Vmicro) in the
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hierarchically porous zeolites were determined by using the αs method (far more appropriate than the t-plot method25, 26); the total pore volume (Vtotal) was obtained from the nitrogen amount adsorbed at the relative pressure of about P/P0 = 0.95. The mesopore
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volume (D> 2nm, Vmeso) was set as the difference between Vtotal and Vmicro. The pore size distributions (PSD) were evaluated from the adsorption isotherms by the Density Functional
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Theory method.27
The global Si/Al ratio was determined by inductive coupled plasma mass spectroscopy (ICPMS) and by energy dispersive X-ray (EDX) analysis using a FEI Quanta 200F microscope at 15 kV.
The morphology of the crystals was observed by scanning electron microscopy (SEM) using a FEG Hitachi S-4800 microscope between 0.5 and 30 kV. The electron tomography experiments were carried out on a JEOL 2100F transmission electron microscope with a field emission gun operating at 200 kV, equipped with a probe corrector and a GATAN Tridiem energy filter. Prior to observation, the zeolite crystals were dispersed in deionized water and then sonicated for several minutes. Up to 5 droplets were 4
ACCEPTED MANUSCRIPT further deposited onto a copper grid covered by a holey carbon membrane rendered hydrophobic by H2/Ar plasma cleaning. Tomography series were acquired by tilting the specimen over a range of ±60°, with an image recorded every 2°. The images were aligned by using the cross-correlation algorithm implemented in the IMod software. The reconstructions have been computed using 10 iterations within the algorithms based on
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algebric reconstruction techniques implemented in the TOMOJ software. The visualization and quantitative analysis of the final volumes have been done by using the ImageJ software. The acidic properties of the samples were studied by Temperature-Programmed Desorption of ammonia (NH3-TPD) using a home-made device equipped with a conductivity cell. The
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samples were pre-treated at 550°C under air flow (30 ml/min) for 8 h. After returning down to 100°C the samples were swept by a N2 flow (30 ml/min) during 10 min. Then a pure flow
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of NH3 at 30 ml/min was adsorbed at 100°C for 15 min. In order to remove NH3 weakly adsorbed on the zeolite, a N2 flow (30 ml/min) was passed through the reactor for 4 h. Finally, desorption of NH3 was monitored in the temperature range of 100-550°C using a heating rate of 10 °C/min. 27
Al magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were
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performed at 104.27 MHz at a spinning speed of 20000 tr.min-1 using a Varian VNRMS 400 spectrometer with 3.2 mm rotors.
2.3 Catalytic evaluation
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The mordenite powders (parent and desilicated) were pressed into wafers, crushed and sieved to obtain particles with diameters of 150-250 µm. Catalytic reactions were conducted
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in a tubular fixed-bed down-flow reactor (6 mm internal diameter) loaded with 1g of catalyst. The catalyst was supported by a porous disk (60µm) and the dead volume was filled with quartz particles of 200-400 µm in size. The catalyst temperature was monitored with a thermocouple placed inside the bed. The catalyst, previously activated in flowing air (60 mL/min at 550°C for 8h) was loaded into the reactor and dehydrated at 180°C for three hours in flowing air. Pure n-heptane was then fed to the system using a HPLC pump (Gilson) until the operating pressure (50 barg) was obtained. The n-heptane flow was then shifted to the reagent feedstock, consisting of a 50/50 mixture of pent-1-ene and n-heptane (both
5
ACCEPTED MANUSCRIPT from Sigma-Aldrich, 99% purity, WHSV: 0.5-2 h-1). Reactor pressure was regulated using an Equilibar back-pressure regulator. Reaction products were analyzed on line by gas chromatography using a DB 2887 capillary column (100% dimethylpolysiloxane, L=10m, ID = 0.53 mm, film thickness = 3 µm). The families of oligomers, nC5=, with n = 2 to 6 , were identified by GC-MS analysis and lumped
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according to the value of n. Total conversion was calculated from the amount of unconverted linear pentenes (pent-1-ene and c+t-pent-2-enes are readily equilibrated by double bond isomerization under the reaction conditions). Carbon balances higher than 98% were obtained in all runs.The degree of skeletal branching of the dimer fraction (n=2) was
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determined by identifying the isomers contained in the mixture of decenes after hydrogenation.
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3. Results Desilication of mordenite
Series of desilication treatments for 30 min at 80°C with NaOH solutions at different concentrations were performed on the zeolite mordenite CBV90A in order to generate mesoporosity in the crystals. The main characteristics, texture, composition and total acidity
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of the parent and desilicated samples are presented in Table 1.
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Table 1.
Desilication of zeolites with intermediate Al contents involves the selective extraction of
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silicon atoms resulting in a decrease of the Si/Al ratio and a significant loss of material at high base concentrations.
Wide-angle XRD patterns (Fig S1) of the parent mordenite CBV90A and of the desilicated samples exhibited the typical diffraction peaks of the mordenite framework. The XRD patterns were barely affected by the desilication treatment, except for the sample desilicated in a 0.4M NaOH solution for which a decrease of the intensity of the diffraction peaks and of the signal-to-noise ratio was noticed, suggesting some loss of long range crystallinity. This effect has also been observed by Paixão et al., van Laak et al., and Li et al.2831
Nevertheless, the XRD patterns do not reveal any signature of an amorphous phase.
6
ACCEPTED MANUSCRIPT N2 adsorption-desorption isotherms of the parent mordenite CBV90A and treated samples are displayed on Figure 1-A. The curve of the parent mordenite shows the presence of mesopores, with a broad size distribution, formed during the dealumination post-treatment of the parent mordenite. The small step at P/P0 = 0.42 in the desorption branch of the isotherm is typical for a cavitation phenomenon indicating that some of the mesopores are
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connected to the crystal surface via restrictions.
The isotherms of the desilicated samples still present a sharp rise at low relative pressures showing preservation of the microporosity whose amount is barely modified when NaOH concentration increases. Concomitantly, the development of mesopores connected to the
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crystals surface increases with increasing NaOH concentration; nevertheless the cavitation signature remains present, though reduced, in all samples.
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Pore size distributions for the parent zeolite (Figure 1-B) calculated on the adsorption branch reveal a narrow size distribution of mesopores centered at 4 nm and a broader contribution of larger pores centered at ca 10 and 40 nm. The mesopores in the desilicated samples exhibit a broad distribution of sizes spreading from 5 to 20 nm, with a maximum at around
Figure 1
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6-8 nm, and a minor contribution of larger mesopores centered at 40-50 nm.
A better view of the textural changes induced upon desilication is gained from the analysis of
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SEM and TEM micrographs of the parent and modified samples (Figure 2, Figures S2 and S3). The parent mordenite CBV90A is constituted of particles with a large distribution of sizes
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(0.3-2 µm) consisting of aggregates of small boulder-like crystals (70-300 nm) with a smooth surface. Examination of the crystals by TEM reveals some textural heterogeneity with zones featuring intracrystalline mesopores with sizes in the range 3-10 nm. Desilication with 0.2M NaOH resulted in some agglomeration of the individual smaller crystals but above all in a severe dissolution of their surface giving rise to a significant surface roughness. At the same time the density and size of the intracrystalline mesopores increased (Figs S2, S3). At even higher NaOH concentrations (0.3 and 0.4 M) the crystal dissolution process was more pronounced. The sample was formed of aggregates (0.2-2 µm) of highly (meso)porous loosely bonded nanoparticles, some of them of very small sizes (5-15 nm) (Figure 2). As a 7
ACCEPTED MANUSCRIPT consequence the external surface area of the crystals increased (Table 1), reaching 100 m²/g for the highly desilicated D-0.4 material. The 3D-TEM cross-section images of the reconstructed volumes of sample D-0.3 are shown in Figure 3. The two images correspond to a numerical cross section in the XY direction of about 1 nm and they are separated by about 60 nm in the Z direction. The 3D TEM
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observation clearly reveals the presence of an intracrystalline 3-dimensional mesopore network, spread over the whole crystal volume, consisting of cavities and channels with sizes in the range 3-15 nm, well connected, at least for the largest ones, to the external surface.
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Figure 2
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Figure 3
The generation of mesopores in high silica-to-alumina zeolites upon treatment by alkaline solution is the result of the dissolution of silica, or silica-rich crystalline domains. As a consequence both Si/Al and Vmicro/Vtotal (VTotal corresponding here to the volume of adsorbed
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nitrogen at P/P0 = 0.95) ratios of the desilicated samples decrease when the severity of the alkaline treatment increases (Table 1). The significant increase of the amount of external surface area, and above all the evolution of the texture of the materials evidenced by electron microscopy, indicate however that the additional (meso)porous volume generated
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is not fully associated with intracrystalline mesopores. In Figure 4 we have represented the linear relationship existing between the micro and mesoporous volumes of the four
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mordenite samples. On a per weight basis, increasing the severity of the alkaline treatment leads to an increase in mesoporous volume of the analyzed material without apparent modification of the microporous volume. This trend is similar to the observation reported by van Laak et al. in the case of the desilication of mordenite crystals under mild alkaline conditions.31 Such a representation, which is the one generally reported in literature, does not give however a realistic picture of the phenomena associated with the desilication process. Indeed, as shown in Figure 4, when taking into consideration the actual yield of the reaction, ie if the different porous volumes are normalized to the initial weight of parent zeolite engaged, under the conditions of our study, the amount of mesopore volume is 8
ACCEPTED MANUSCRIPT barely affected by the severity of the desilication treatment. Such a phenomenon can be readily explained at the light of the desilication efficiency descriptor introduced by Verboekend et al.32 which relates the mesopore area and volume generated to the mass of zeolite dissolved and the size of the individual crystals. As the alkaline treatment results not only in the generation of intracrystalline mesopores but also in the roughening of the zeolite
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surface, increasing the severity of the treatment leads to a fragmentation of the crystals. When the size of the crystal fragments approaches that of the intracrystalline mesopores, the generation of additional mesoporosity becomes less (or no longer) efficient as the main process involved is the dissolution of the nanosized zeolite crystals. In the present study it is
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clear that the alkaline treatment performed with a 0.2M solution of sodium hydroxide leads essentially to surface roughening besides minimal creation of internal mesoporosity. At
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higher base concentrations crystal fragmentation and dissolution are the major processes taking place, no formation of additional intracrystalline mesopores can be detected.
Figure 4
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Acidity
The NH3-TPD profile of the parent mordenite CBV90A (Figure 5) shows two broad peaks, a small one at low temperature (at about 210°C) associated with weak acid sites and a more intense one (80% of the total acidity) at high temperature (around 510°C) associated with
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strong acid sites. The TPD profiles of the desilicated samples (Figure 5-A, Table 1) show that the total number of acid sites per unit of weight increases as NaOH concentration increases
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due essentially to an increase of the number of weak acid sites. This increase is accompanied by a slight shift of the desorption temperature to higher values (from 210 to 310°C).
Figure 5
The increase of the total number of acid sites per unit of weight is consistent with the decrease in Si/Al ratio, as the material becomes richer in Al, a phenomenon also reported by Groen and co-workers33 and X. Wei and P.G. Smirniotis34, for example. However, the increase in the number of acid sites is not strictly proportional to the increase in aluminium content of the sample as seen in Table 2. While in the starting material 90% of the Al atoms 9
ACCEPTED MANUSCRIPT of the sample generate an acid site, nearly half of the Al atoms do not contribute to acidity on the highly desilicated sample.
Table 2. Moreover, if the TPD profiles are normalized to the initial amount of zeolite (Figure 5-B) a
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somewhat different picture is obtained as it clearly appears that, in fact, the number of weak acid sites is barely maintained constant after desilication while the number of strong acid sites is dramatically decreased. Desilication results therefore in the removal of strong acid sites through the dissolution of the zeolitic framework and the disappearance of
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tetrahedrally coordinated aluminium. Actually, in the more severely desilicated sample a flat distribution of acid strengths is observed indicative of a highly faulted zeolite network.
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This conclusion is supported by the 27Al NMR spectra of the samples (Figure 6). The parent material exhibits a single signal at 54 ppm indicating that most of the Al is located in the framework in tetrahedral sites and prone to generate strong acid sites in agreement with the quantitative titration using ammonia. In the desilicated samples a new signal characteristic of aluminum atoms in octahedral sites not covalently bonded to the zeolite lattice appears at
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0 ppm and the linewidth of the peak at 54 ppm broadens, indicating a less symmetrical environment of the lattice tetrahedrally bonded Al atoms.
Catalytic activity
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The catalytic behavior of two samples, namely the parent mordenite CBV90A and the desilicated D-0.2 sample, has been investigated in the oligomerization of pent-1-ene at
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180°C under 50 barg of pressure and a WHSH of 1 h-1. These reaction conditions have been selected on the bases of a series of preliminary experiments performed at varying temperatures and space velocities (Figures S5 and S6). Under the conditions of the test the two catalysts were very active, leading to initial conversions above 95%. The reaction products consisted solely in mixtures of nC5= oligomers, with n comprised between 2 and 6. No evidence for the occurrence of cracking reactions was obtained (Figure 7). Detailed analysis of the composition of the hydrogenated dimer fraction (C10) on both catalysts at different times on stream was achieved in order to determine the degree of branching of the C10 oligomers. This revealed the absence of isomerization reactions leading to a change in 10
ACCEPTED MANUSCRIPT the branching of the hydrocarbon chain (Type B rearrangements) under the conditions of the reaction. The obtained products consisted of 60 ± 2% monobranched hydrocarbons (>90% methyl-nonenes and ethyl-octenes), 38 ± 2% dibranched hydrocarbons ( 80% dimethyloctenes, 20% diethyl-hexenes plus methyl-ethyl heptenes) with only marginal amounts ( < 2%) of linear decenes. Such a product distribution is well accounted for by classical
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alkylcarbenium ion chemsitry involving addition of the secondary carbenium ions generated from pent-1-ene and pent-2-enes to the parent olefins followed by c:t and double bond migration and eventually alkyl shifts (Type A rearrangement) and ultimately, deprotonation35. As regards the catalyst stability and selectivity of the oligomer products,
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the effect of the generation of a secondary network of mesopores by desilication, which generates external surface area and improves mass transfer inside zeolite crystals is clearly
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apparent from Figure 7 and table 3. Stability of the activity with time on stream was significantly improved and the oligomers distribution was shifted towards heavier (C15-C30) products in the D-0.2 sample. Both effects result from enhanced mass transfer and better desorption of the heavier oligomers, and of the poison precursors, from the zeolite crystals featuring a high density of mesopores22,36 due to shorter path lengths.37,38 The catalyst
4. Discussion
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demonstrated moreover high stability with regards repeated reaction/regeneration cycles.
In this study the desilication of mordenite crystals, previously enriched in silica by
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dealumination, at increasing severity of the alkaline treatment has been followed using different characterization techniques and analyzed quantitatively in order to reach a clear
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and realistic description of the phenomena involved in the process of mesopore formation. On the basis of our observations it can be concluded that the generation of mesoporosity starts by the hydrolysis and dissolution of silicate species near the surface of the crystals as already suggested by van Laak et al.31 This process is more probably initiated at defect sites, and results in a progressive surface roughening and creation of some intracrystalline mesoporosity, followed by fragmentation and, ultimately, in a complete dissolution of the crystal. Interestingly, the estimation of the mesopore volume created relative to the weight of the initial zeolite brings to the conclusion that most of the material removed from the crystal by dissolution does not contribute to the formation of intracrystalline mesoporosity. As a consequence of this dissolution process, the development of mesoporosity in the 11
ACCEPTED MANUSCRIPT mordenite crystals is accompanied by a severe loss of zeolitic crystalline material and of the strong acidity associated to it. The observed dissolution figures, namely the surface roughening and the shapes of the intracrystalline mesopores, cannot be interpreted on the bases of the structural features of the mordenite framework but more probably by considering the composition of the zeolite
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and, more precisely the distribution of the tetrahedral aluminium sites in the lattice. Such a statement is in line with the well known influence of structural aluminium in controlling zeolite dissolution.
Regarding crystal morphology changes reported here for mordenite, our observations
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closely resemble those made by Verboekend et al.32 in the case of the desilication of nanometric rod-like ZSM-22 and by Bonilla et al.33 for the desilication of aggregates of thin
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platelets of ferrierite but contrast with those reported in the desilication of large mordenite33,34 and ZSM-540 crystals in NaOH solution. In the latter cases the alkaline attack allowed generating high mesoporosity and external surface while preserving the original morphology of the crystals.
Similar contrasting behaviors have been pointed out and tentatively rationalized by correlating the amount of mesopores or external surface generated and the amount of
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zeolite dissolved to the initial size of the crystals, as said above.32 Actually, the occurrence of non uniform dissolution patterns is a rather common phenomenon observed during the chemical weathering of rocks, including silicate minerals, and can be extended to the
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dissolution of zeolites as we reported some years ago41 in a study dealing with the crystallization and subsequent dissolution of mazzite in various media. Zeolite crystals in a
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highly undersaturated medium dissolve via the formation of etch pits on all exposed surfaces. When this occurs, etching takes place preferentially at dislocations or defects that intercept the crystal surface. Low index, flat, crystal faces are less affected because of their lower density of defects resulting from their lower growth rates. Dissolution by etching occurs therefore essentially on high index faces, at corners or edges, on atoms with lower coordination. The dissolution rate of the various faces determines crystal habit, as it is the case for crystal growth. It is therefore clear that crystal morphology, and more generally speaking the history of the parent crystal submited to desilication by alkaline attack, can play
12
ACCEPTED MANUSCRIPT a role as important as composition and structure in determining the outcome of the desilication process. As regards the catalytic behavior of the desilicated mordenite, the generation of intracrystalline mesopores and the decrease of the size of the crystals result in a significant improvement of the stability and of the activity of the catalyst in the oligomerization of
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linear pentenes and a shift of the product distribution towards heavier oligomers, which is well in line with an easier desorption of the reaction intermediates from the acidic sites.
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5. Conclusion
Quantitative analysis of the textural, composition and acidity changes occurring during the
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desilication of zeolite crystals, previously enriched in silicon by dealumination, using NaOH solutions with increasing base concentrations allowed us to draw a consistent scheme of the mechanism of mesopore formation. The generation of mesoporosity is due to the hydrolysis and dissolution of the surface of the crystals, probably initiated by the formation of etch pits at low coordination atoms, leading to surface roughening, formation and growth of internal mesopores followed by fragmentation and ultimately, by a complete dissolution of the
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crystal. Such a process results in a severe loss of crystalline material and of the strong acidity associated to it. As a consequence of an easier intracrystalline mass transport and a decrease in particle size, the catalytic activity of the desilicated mordenite in the
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oligomerization of pentenes was significantly improved.
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Acknowledgements
TOTAL Research and Technology Center of Feluy (Belgium) is acknowledged for funding the present study. C. Bertrand-Drira is especially thankful to TOTAL for her grant. References
[1] W. Vermeiren, J.P. Gilson, Topics in Catalysis 52 (2009) 1131-1161. [2] S. van Donk, A.H. Janssen, J.H. Bitter, K.P. de Jong, Catal. Rev. 45 (2003) 297-319. [3] J. Perez-Ramirez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen, Chem. Soc. Rev. 37 (2008) 2530-2542. [4] C.H. Christensen, K. Johannsen, E. Tornqvist, I. Schmidt, H. Topsoe, C.H. Christensen, Catal. Today 128 (2007) 117-122. 13
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[5] C.C. Freyhardt, M. Tsapatsis, R.F. Lobo, K.J. Balkus, E. Davis, Nature 381(1996) 295-298. [6] A. Corma, M.J. Díaz-Cabañas, J.L. Jordá, C. Martínez, M. Moliner, Nature 443 (2006) 842845. [7] M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, R. Ryoo, Nature 461 (2009) 246-249. [8] L. Tosheva, V.P. Valtchev, Chem. Mater. 17 (2005) 2494-2513. [9] F.N. Gu, F. Wei, J.Y. Yang, N. Lin, W.G. Lin, Y. Wang, J.H. Zhu, Chem. Mater. 22 (2010) 2442-2450. [10] R. Chal, C. Gérardin, M. Bulut, S. van Donk, Chem. Cat. Chem. 3 (2011) 67-81. [11] Y.S. Tao, H. Kanoh, L. Abrams, K. Kaneko, Chem. Rev. 106 (2006) 896-910. [12] K.R. Kloetstra, H. Van Bekkum, J.C. Jansen, Chem. Commun. (Camb) (1997) 2281-2282. [13] K.R. Kloetstra, H.W. Zandbergen, Microporous Materials 6 (1996) 287-293. [14] B.T. Holland, L. Abrams, J. Amer. Chem. Soc. 121 (1999) 4308-4309. [15] Y. Liu, W. Zhang, T.J. Pinnavaia, J. Amer. Chem. Soc.122 (2000) 8791-8792. [16] I.I. Ivanova, E.E. Knyazeva, Chem. Soc. Rev. 42(9) (2013) 3671-3688. [17] R. Chal, T. Cacciaguerra, S. van Donk and C. Gérardin, Chem. Commun. 46 (2010) 7840-7842 [18] G. Bellussi, F. Mizia, V. Calemna, P. Pollesel, R. Millini, Microp. Mesop. Mater. 164 (2012) 127-134. [19] R. Schmidt, M.B. Welch, B.B. Randolph, Energy & Fuels 22 (2008) 1148-1155. [20] Ch. Marcilly, Catalyse acido-basique, Application au raffinage, Vol.2, Editions Technip, Paris, 2003, pp 475-506 [21] J.P.G. Pater, P.A. Jacobs, J.A. Martens, J. Catal. 179 (1998) 477-482. [22] J.P.G. Pater, P.A. Jacobs, J.A. Martens, J. Catal. 184 (1999) 262-267.
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[23] S. Brunauer, P.H. Emmett, E. Teller, J. Amer. Chem. Soc. 60 (1938) 225-227. [24] M.R. Bhambhani, P.A. Cutting, K.S.W. Sing, D.H. Turk, J. Coll. Interf. Sci. 38 (1972) 109117. [25] M. Robitzer, F. Di Renzo, F. Quignard, Microp. Mesop. Mater. 140 (2011) 9-16 [26] A. Galarneau, F. Villemot, J. Rodriguez, F. Fajula, B. Coasne, Langmuir 30 (2014) 1326613274 [27] A.V. Neimark, P.I. Ravikovitch, Microp. Mesop. Mater. 44-45 (2001) 697-707. [28] V. Paixão, A.P. Carvalho, J. Rocha, A. Fernandes, A. Martins, Microp. Mesop. Mater. 131 (2010) 350-357. [29] V. Paixão, R. Monteiro, M. Andrade, A. Fernandes, J. Rocha, A.P. Carvalho, A. Martins, Appl. Catal. A-Gen 402 (2011) 59-68. [30] X. Li, R. Prins, J.A. van Bokhoven, J. Catal. 262 (2009) 257-265. [31] A.N.C. van Laak, S.L. Sagala, J. Zecevic, H.Friedrich, P.E.de Jongh, K.P. de Jong, J. Catal. 276 (2010) 170-180 [32] D. Verboekend, A.M. Chabaneix, K. Thomas, J.-P. Gilson, J. Pérez-Ramírez, CrystEngComm 13 (2011) 3408-3416. 14
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[33] J.C. Groen, T. Sano, J.A. Moulijn, J. Pérez-Ramirez, J. Catal. 251 (2007) 21-27. [34] X. Wei, P.G. Smirniotis, Microporous and Mesoporous Materials 97 (2006) 97-106. [35] G.A. Olah, A. Molnar,. Hydrocarbon Chemistry, 2nd ed.; Wiley-Interscience: New York, 2003. [36] A. Corma, C. Martinez, E. Doskocil, J. Catal., 300 (2013) 183- 196. [37] S. van Donk, A. Broersma, OLJ. Giizeman, J.A. von Bohkoven, J.H. Bitter, K.P. de Jong, J. Catal. 204 (2001) 272-280 [38] M. Tromp, J.A. von Bohkoven, M.T.Garriga Oostenbrink, J.H. Bitter, K.P. de Jong, D.C. Koningsberger, J. Catal. 190 (2000) 209-214 [39] A. Bonilla, D. Baudouin, J. Pérez-Ramirez, J. Catal. 2065 (2009) 170-180. [40] J.C. Groen, W. Zhu, S. Brower, S.J. Huynink, F. Kapteijn, J.A. Moulijn, J. Pérez-Ramirez, J. Amer. Chem. Soc. 129 (2007) 355-360. [41] F. Di Renzo, F. Fajula, F. Figueras, S. Nicolas, T. Des Courieres, Stud. Surf. Sci. Catal. 49 (1989) 119-132.
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Table 3. Conversion and oligomer selectivity as a function of catalyst and time on stream (Same conditions as in Figure 7) CBV90A
D-0.2 70 h 78 0.37 0.41 0.22
40h 95 0.10 0.29 0.61
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20h 89 0.25 0.43 0.32
75 h 94 0.13 0.32 0.55
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Catalyst Time on stream (h) Pentene conversion (%) C10= C15= C20= - C30=
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Mesoporous mordenites obtained by desilication: mechanistic considerations and evaluation in catalytic oligomerization of pentene
Chloé Bertrand-Drira1, Xiao-wei Cheng1, Thomas Cacciaguerra1, Philippe Trens1, Georgian
1
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Melinte2, Ovidiu Ersen2, Delphine Minoux3, Annie Finiels1, François Fajula1, Corine Gerardin1* Institut Charles Gerhardt, UMR 5253 UM2-CNRS-ENSCM-UM1- Matériaux Avancés pour la
Catalyse et la Santé, ENSCM 8 rue de l’Ecole Normale, 34095 Montpellier Cedex, France Institut de Physique et Chimie des Matériaux, UMR 7504 CNRS, Strasbourg, France
3
Total Research & Technology Feluy, Belgium
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2
Highlights :
Desilication generates surface roughening and inter-and intracystalline mesopores. Material dissolution does not necessarily lead to intracrystalline mesopores.
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Desilication process resembles chemical weathering of rocks.
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Catalytic pentene oligomerization is largely improved with desilicated mordenites.
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Figure captions
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Figure 1: A. N2 adsorption-desorption isotherms of the parent mordenite CBV90A and desilicated samples, B. Pore size distributions. Figure 2: SEM (left) and TEM (right) images of the parent mordenite CBV90A and of the D- 0.3 desilicated sample
correspond to slices separated by about 60 nm
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Figure 3. Cross-section images of 3D-TEM reconstructed volumes of D-0.3 sample. The two images
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Figure 4. Two presentations of the relationship between the micro and mesoporous volumes in desilicated mordenites (per gram of analyzed material (diamonds), and per gram of initial zeolite (triangles))
Figure 5. NH3-TPD spectra of the parent mordenite CBV90A and of the desilicated samples. A: per gram of analysed material, B: per gram of initial zeolite
Figure 6. 27Al MAS NMR spectra of parent mordenite and D- 0.2 and D- 0.4 desilicated samples
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Figure 7. Oligomerization of pentene over the parent CBV90 (diamonds) and desilicated D-0.2 (squares) catalysts at 180°C, 50 barg, WHSV = 1h-1. Inset: typical chromatogram with oligomer distribution obtained over D-0.2 catalyst.
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Supplementary information : Figure Captions Figure S1: XRD patterns of the parent and desilicated samples of mordenite
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Figure S2: Scanning Electron Micrographs of the parent and desilicated samples of mordenite Figure S3: Transmission Electron Micrographs of the parent and desilicated samples of mordenite Figure S4. A. 3D surface model of the selected D-0.3 zeolite grain; B. Longitudinal (XY) slice through the 3D model of the selected D-0.3 zeolite grain. Figure S5: Catalytic activity of catalyst D-0.2 for pentene oligomerization at different temperatures Figure S6: Influence of space velocity on the stability of the activity (catalyst D-0.2)
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Table 1. Porosity and acidic properties of the parent and desilicated mordenites at different NaOH concentrations and their corresponding Si/Al and yields of the desilication treatments Vmicro c (ml g-1)
Vmeso d (ml g-1)
CBV90A
588
50
0.20
0.08
D-0.2
543
77
0.18
0.17
D-0.3
591
89
0.18
D-0.4
542
103
0.17
Sample
Vtotal e
Yield
(ml g-1)
Si/Al f (at/at)
NH3 TPD (mmol g-1)
(wt%)
0.28
48
0.31
/
0.35
35
0.39
57
0.29
0.47
24
0.45
37
0.37
0.54
16
0.54
21
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Sextb (m² g-1)
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SBET a (m² g-1)
BET method, b αs method, c Volumes determined using αs method , d difference between Vtotal and Vmicro, e Volume adsorbed at P/P0=0.95 on the N2 adsorption branch, f EDX.
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a
Table 2. Total acidity and amount of weak and strong acid sites of the mordenite parent and desilicated samples
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Si/Al
Sample
Al/(Al+Si)
mmol Al/g
Total acidity mmol.g-1
Weak acid sites mmol.g-1
Strong acid sites mmol.g-1
(at/at)
CBV90A
48
0.0204
0.34
0.31
0.06 (20%)
0.25 (80%)
D-0.2
35
0.0278
0.46
0.39
0.14 (35%)
0.25 (65%)
D-0.3
24
0.0407
0.68
0.45
0.18 (39%)
0.27 (61%)
D-0.4
16
0.0584
0.98
0.54
0.25 (46%)
0.29 (54%)
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A
D-0.4 400
D-0.3 D-0.2 CBV90A
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Vads / ml.g-1
350
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450
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Figure 1A
300 250 200 150
50 0
0.2
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0
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100
0.4
0.6 P/P0 / -
0.8
1
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0.07
B
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Figure 1B
D-0.4 D-0.3
0.06
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PSD / ml.g-1.nm -1
D-0.2
0.05 0.04 0.03
0.01 0
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0.02
AC C
EP
1
10 Pore diameter / nm
CBV90A
100
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Figure 2
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RI PT
Figure 3
ACCEPTED MANUSCRIPT
RI PT
Figure 4 0.25
0.2
CBV90A
D-0.2
0.1
0 0
0.05
TE D
D-0.4
0.1
0.15
0.2
SC 0.25
Vmeso / mL.g-1
EP
D-0.4
Normalization to the initial amount of zeolite
D-0.3 0.05
D-0.3
M AN U
0.15
AC C
Vmicro / mL.g-1
D-02
0.3
0.35
0.4
Figure 5A
12
TE D
D-0.4
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
D-0.3 D-0.2
CBV90A
EP
8 6 4
AC C
Intensity / u.a.
10
2 0
100
200
300 400 500 Temperature / °C
600
700
ACCEPTED MANUSCRIPT
12
RI PT
Figure 5B
D-0.4 D-0.3
10
SC
Intensity / u.a.
D-0.2 8
4 2 0 200
300 400 500 Temperature / °C
AC C
EP
TE D
100
M AN U
6
600
700
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 6
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure 7
ACCEPTED MANUSCRIPT
D-0.4
SC
Intensity / u.a.
D-0.3
RI PT
Figure S1
M AN U
D-O.2
CBV90A
10
15
20
25 30 35 2 θ / degrees
AC C
EP
TE D
5
40
45
50
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure S2
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure S3
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
Figure S4
ACCEPTED MANUSCRIPT
Figure S5
9
150°C 50barg WHSV=1h-1
180°C 50barg WHSV=1h-1
90
SC
70 60 50
Sélectivités C10 : 0,11 C15 : 0,30 C20+ : 0,59
Sélectivités C10 : 0,26 C15 : 0,36 C20+ : 0,38
150°C 50 barg WHSV = 1 h-1
30 20
180°C 50 barg WHSV = 1 h-1
M AN U
40
kD=0,0002 h-1
kD =0,0098 h-1
10 0 10
20
30
40
TE D
0
EP
TOS (h)
AC C
Conversion (%)
80
G-D90-0,2Na-0,5 Si/Al=35 Acidité=0,41 mmol/g Vmicro=0,19 ml/g VLméso=0,21ml/g
RI PT
100
8
50
60
70
80
ACCEPTED MANUSCRIPT
Figure S6 1
2
180°C 50barg WHSV=1h-1
RI PT
180°C 50barg WHSV=2h-1
WHSV=1h
100 90
SC
Sélectivités C10 : 0,19 C15 : 0,49 C20+ : 0,32
70 60
180°C 50 barg WHSV = 1 h-1
50 40
Sélectivités C10 : 0,18 C15 : 0,41 180°C C20+ : 0,41
50 barg WHSV = 1 h-1
M AN U
Conversion (%)
80
Sélectivités 180°C C10 : 0,27 50 barg C15 : 0,41 -1 WHSV C20+ : 0,32= 2 h
30 20
kD =0,0005h-1
10
TE D
kD =0,0015h-1 kD =0,0002h-1
0 0
30
Régénération 500°C 8h air
60
AC C
EP
TOS (h)
90
Arrêt chauffage pression
120