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Synthesis of hierarchical zeolite templated carbons

T Aumond, J Rousseau, Y Pouilloux, L Pinard, A Sachse

To cite this version:

T Aumond, J Rousseau, Y Pouilloux, L Pinard, A Sachse. Synthesis of hierarchical zeolite templated

carbons. Carbon Trends, Elsevier, 2021, �10.1016/j.cartre.2020.100014�. �hal-03106784�

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Contents lists available at ScienceDirect

Carbon

Trends

journal homepage: www.elsevier.com/locate/cartre

Synthesis

of

hierarchical

zeolite

templated

carbons

T.

Aumond,

J.

Rousseau,

Y.

Pouilloux,

L.

Pinard,

A.

Sachse

Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), Université de Poitiers – UMR 7285 CNRS, UFR SFA, Bat. B27, 4 rue Michel Brunet, TSA 51106, 86073 Poitiers, Cedex 9, France

a

r

t

i

c

l

e

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Article history: Received 19 October 2020 Revised 19 November 2020 Accepted 20 November 2020 Keywords:

Zeolite templated carbons ZTC Hierarchical Carbon Microporous Mesoporous Zeolite Surfactant-templating

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b

s

t

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a

c

t

Theachievementofhierarchicalzeolitetemplatedcarbons(ZTCs) isfirstlyreportedthroughtheuseof mesoporouszeolitesassacrificialscaffolds.TheachievedhierarchicalZTCsfeaturebothtailored microp-orosityandhighdegreeofmesoporosityandallowhencetobridgethegapbetweenclassicalZTCsand CMK-likematerials.Mesoporosity fromsteamedzeoliteswas meticulouslytranscribedtotheZTC parti-cles.Theuseofzeolitesfeaturingsurfactant-templatedmesoporosity allowedforachievingaZTCwith averyhighmesoporevolume(0.85cm3g−1).Therearrangementofsurfactant-templatedmesoporosity

could beevidenced throughnitrogenphysisorption and transmissionelectronmicroscopy on ultrami-crotomedsamples.TherearrangementofmesoporosityduringZTCformationhasbeenfoundtoimpact coherentcrystalsizeandthelatticestrainofthezeolitetemplate.ThecombinationofdatafromX-ray diffractionandnitrogenphysisorptionallowedfurthertorevealthatthethicknessoftheZTCskeletonis impactedbythedegreeofintracrystallinemesoporosityinthetemplatezeolite.Throughcomparingthe texturalcharacteristicsofthezeolitetemplatesandoftheachievedhierarchicalZTCs,importantevidence wasfoundthatthesurfactant-templatingofUSYzeoliteleadstothedevelopmentofasecondary amor-phousphaseandthatthereductionofthemicroporousvolumeduringthisprocessismerelyapparent.

© 2020TheAuthor(s).PublishedbyElsevierLtd. ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/)

1. Introduction

Nano-casting technologies have revealed as very powerful strategies for the design of carbon materials featuring tailored tex- tural properties [1] . Prominent examples of ordered mesoporous carbons, such as the CMK family, are achieved through the use of mesoporous silica materials ( e.g. MCM-48, SBA-15, etc.) as scaf- folds. Generally the inorganic mold is padded with the carbon source (furfuryl alcohol, sucrose or phenol resin), which subse- quently polymerizes prior to pyrolysis. After the dissolution of the mold the pure carbon is achieved that features the negative textu- ral properties of the scaffold.

The same concept was proposed in the quest of microporous carbons with tailored microporosity through the use of zeolites as sacrificial scaffolds. Zeolites feature characteristic voids that through their interconnection lead to the development of extended microporous systems of defined topologies. First attempts to use zeolites as molds employed bulky molecules as carbon sources and failed to transcribe the microporous texture to the final carbon ma- terials, as the precursors did not allow to perfectly fill-up the ze- olite void spaces [2] . Smaller gaseous carbon precursors, such as

Corresponding author.

E-mail address: alexander.sachse@univ-poitiers.fr (A. Sachse).

ethylene and acetylene, which can freely diffuse throughout the extended zeolite microporous systems have revealed very efficient for the development of zeolite templated carbons (ZTCs) of high structural quality [ 3 , 4 ].

ZTCs are today a well-established class of microporous carbons that feature the textural and morphological properties of the tem- plate zeolites. The impact of various parameters of the zeolite tem- plate on the ZTC formation have been studied and are mostly re- lated to the framework type and composition [4] . The most suited zeolite frameworks to act as template for the ZTC synthesis have revealed to be the FAU, EMT and ∗BEA structures, due to their large voids ( >0.6 nm) and 3 dimensional interconnected micropore sys- tems.

ZTCs differ to classically activated carbons, by their defined microporous topologies, which allows them to act as model structures. Activated carbons, which are industrially exploited as support materials and as adsorbents, present highly disordered structures of cross-linked, randomly packed graphene sub-units connected through amorphous carbon parts [5] . This latter hence feature random slit-like micropores. Future studies will indicate if the relative high cost of ZTCs production is susceptible to be over- powered by the impact that defined porous architectures might feature in energy storage devices [6] .

https://doi.org/10.1016/j.cartre.2020.10 0 014

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T. Aumond, J. Rousseau, Y. Pouilloux et al. Carbon Trends 2 (2020) 10 0 014 The ability to tailor the textural and morphological properties

of micropores materials has important implication of the diffusion path length (DPL) [7] . Shortening the DPL allows for increasing the effectiveness factor of many catalytic processes involving micro- porous materials [8] . The most straightforward strategy to tailor the DPL is to modify the particle size. Large particles will present an extended DPL, while small particles will feature short DPL and higher effectiveness factors in a given catalytic processes. Despite the emphasis that tailoring textural properties in ZTC applications could present this aspect has only scarcely been approached in ZTC synthesis. Garsuch et al. [9] compared the quality of ZTCs obtained using Y zeolite templates (FAU structure) using small (0.5 μm) and large (3 μm) crystals. The authors observed higher microporous volume and increased long range order employing the small crys- tals as templates and concluded that for the larger crystals diffu- sion issues of the ethylene precursor may arise that would lead to a fraction of unfilled pores in the center of the large crystals. It is yet to note that the Si/Al ratio of the two template zeo- lite was different. Unfortunately this latter circumstance was not taken into consideration for explaining the different templating ability. Valtchev and co-workers studied the impact of the crystal size of beta zeolite ( ∗BEA structure) on the properties of achieved ZTCs [10] . The authors compared the templating efficiency of three beta zeolites featuring small (0.1 μm), medium (0.5 μm) and large (1 μm) crystals and observed that ZTCs with highest structural or- dering were achieved using the medium-sized zeolite as template. Here again the possible effect of the Si/Al ratio was not taken into consideration.

A further very powerful strategy to tune the textural proper- ties of materials is the development of porosity on various length scales [11] . Such hierarchical porous systems prove advantageous in many applications such as adsorption and catalysis; due to the extreme reduction of the DPL [12] . Hierarchical microporous ma- terials are those that next to their intrinsic microporosity feature secondary larger porosity (mesopores or macropores). Such sec- ondary porosity is classically generated by top-down or bottom-up approaches [13] .

Zeolite hierarchization is an industrially relevant process and the paradigm is the dealumination of zeolite Y [14] . During this dealumination process the zeolite is exposed to water vapor at el- evated temperature (steaming), which allows to partially eliminate aluminum from the framework leading to the commonly named USY (Ultra Stable Y) zeolite. USY features higher thermal resistance due to the reduction of the framework Al concentration, crucial for its utilization as component in the fluid catalytic cracking (FCC) catalyst. The steaming process further leads to the development of intracrystalline mesopores [15] . It is these mesopores that allow for a beneficial impact in cracking reactions, as they reduce the diffu- sion path length. This latter is of upmost importance as it allows for minimizing the occurrence of secondary reactions that lead to reduced gasoline yield and increased catalyst deactivation in FCC [16] .

The development of ZTCs featuring hierarchical porosity has so far not been described. Here we firstly report the develop- ment of hierarchical ZTCs through a bottom-up approach by us- ing mesoporous zeolite templates. The achievement of hierarchi- cal ZTCs is susceptible to greatly improve their textural properties through shortening diffusion path lengths. Such systems are hence very promising for increasing their performances in hydrogen and methane storage, in heterogeneous catalysis and for the develop- ment of advanced energy transformation systems [ 4 , 17 ]. The par- ticular challenge when using hierarchical zeolites as templates of ZTCs is to achieve the thorough transcription of their textural fea- tures and in particular to avoid the formation of non-templated carbon during the synthesis.

2. Experimentalsection

2.1. Materials

Sodium hydroxide (Sigma-Aldrich, 99%), cetyltrimethylammo- nium bromide, (CTAB, Sigma-Aldrich, 98%), ammonium nitrate (Sigma-Aldrich, 99%), CBV720 (Zeolyst, hereafter named Z-USY), hydrofluoric acid (48% in water, Sigma-Aldrich), sodium bicarbon- ate (Sigma-Aldrich, 99%), boric acid (Sigma Aldrich, 99%) ethylene (Air liquid, 99.5%) and nitrogen (Air liquid, 99.995%) were pur- chased and used as received.

2.2. Synthesisofsurfactant-templatedUSY(USYST)

Surfactant-templated USY (Z-USY ST) was synthetized by dissolv-

ing CTAB (0.5 g) in 20 mL of an aqueous NaOH solution (0.09 M) in a Teflon autoclave. CBV720 (1 g) was then added and mixt for 5 min at RT. The autoclave was sealed, transferred to an oven and heated at 80 °C for 8 h. The autoclave was cooled to RT, the solid recovered through filtration and washed with distilled water un- til neutral pH. The solid was dried at 80 °C during 12 h and cal- cined at 550 °C in air for 4 h. The obtained material was proton ex- changed using 100 mL of an aqueous 1 M NH 4NO 3at 80 °C under

stirring for 1 h. After filtration and washing with distilled water the exchange cycle was repeated two times, dried at 80 °C during 12 h and calcined at 550 °C in air for 4 h. The achieved material was named Z-USY ST.

2.3. Synthesisofhierarchicalzeolitetemplatedcarbons(C-USYand C-USYST)

The synthesis strategy of ZTCs is depicted in Scheme 1 . Two FAU based zeolite samples featuring different textural proper- ties (Z-USY and Z-USY ST, the Z stands for zeolite) were used

as template. The template zeolite (2 g) was placed on a frit in a cylindrical quartz reactor and activated under nitrogen flow (150 mL min −1) at 690 °C for 1 h. The activation step is required to remove the amount of physisorbed water in the samples. Ethylene (6.7%) was injected to the flow during 4 h at this temperature after which the temperature was gradually risen to 790 °C under pure nitrogen flow and kept for 2 h. After cooling to RT the obtained black powder corresponds to the hybrid materials (carbon/zeolite) named H-USY and H-USY ST (the H stands for hybrid). The hybrid

materials were further treated with 20 mL HF in a Teflon beaker at RT during 4 h. After neutralization with 120 mL of a saturated aqueous solution of boric acid and 120 mL of a saturated aque- ous solution of NaHCO 3 a black solid was recovered through filtra-

tion and washed with 500 mL distilled water. After drying at 80 °C during 12 h the pure carbon materials were achieved and named C-USY and C-USY ST(the C stands for carbon).

2.4. Characterization

Nitrogen physisorption measurements were carried out using a Micromeritics 3FLEX at -196 °C. Approximately 50–60 mg of sample was out-gassed prior to each experiment during 12 h at 350 °C. The total pore volume was determined from the nitrogen adsorbed volume at p/p0 = 0.95. The micropore volume was cal- culated through t-plot method. The thickness of the nitrogen layer was calculated using reference isotherms of ( i) a non-porous sil- ica (used for the t-plot of the template zeolites, TableS1) and ( ii) graphene (used for the t-plot of the ZTCs, Table S2). The meso- pore volume was determined by the difference between the total pore volume and the micropore volume. The pore size distributions were determined using density functional theory (DFT) models. For

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Fig. 1. a) Nitrogen adsorption and desorption isotherms at 77 K and b) DFT pore size distribution for Z-USY (black full symbols) and Z-USY ST (blue empty symbols). It is to note that DFT pore size distribution does not allow for presenting the steamed mesopores (uptake p/p 0 > 0.8) for Z-USY.

USY and USY ST a DFT model based on cylindrical pores in oxide

materials was used. For C-USY and C-USY STa model based on slit-

like pores in carbon materials was employed. The indicated pore size does not represent an absolute value and strongly depends on the used model. Comparison of pore sizes based on these models should hence be made with caution.

Powder diffractograms were collected using a PANalytical Empyrean X-ray diffractometer using CuK

α

radiation (1.54059 ˚A), for 2

θ

ranging from 5 ° to 40 °. Scan speed was fixed at 0.008 ° min −1. The Willamson-Hall relation was used to calculate coherent crystallite size (CCS or D) and the lattice strain ( ɛ): [18]

β

hklcos

θ

= K

λ

D +4

ε

sin

θ

where

β

hkl is the full width of half maximum (FWHM), K stands for the shape factor (0.9),

λ

represents the wavelength of CuK α radiation. The lattice parameter (a 0) was calculated through Ri-

etveld refinement of the diffractograms using the Highscore plus software.

Transmission Electronic Microscopy (TEM) coupled with Energy Dispersive X-ray Spectroscopy (EDXS) was performed using a JEOL 2100 instrument (operated at 200 kV with a LaB 6 source and

equipped with a Gatan Ultra scan camera).

Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-790CF microscope.

Raman spectroscopy was carried out using a Raman HORIBA JOBIN YVON Labram HR800UV confocal microscope equipped with a Peltier cooled CCD detector. The exciter wavelength was 532 nm. A diffraction grating with 600 lines mm −1 and a confocal hole of 200

μ

m was applied.

Thermogravimetric analysis was performed using a SDT Q600 by heating the samples in a N 2/O 2 (4:1) gas stream

(100 mL min −1) with a ramp of 10 °C min −1up to 900 °C. The Si/Al ratio of the template zeolites was determined using ICP-OES analysis, through using a Perkin Elmer Optima 20 0 0 DV instrument.

3. Resultsanddiscussion

3.1. Characterizationofthehierarchicalzeolitetemplates

Two mesoporous zeolites were used as templates for the syn- thesis of hierarchical ZTCs. These were USY and the surfactant- templates USY, named Z-USY and Z-USY ST, respectively. For Z-USY

a type I nitrogen physisorption isotherm was achieved, character- istic for microporous materials ( Fig. 1a). The increase in nitrogen

uptake at high relative pressures ( > 0.8) indicates the presence of larger mesopores. The sample additionally presents some small mesopores centered at 2 nm ( Fig. 1b). The observed H4 hystere- sis loop is typical for zeolites presenting some larger mesopores [19] . The microporous and mesoporous volume amount to 0.29 and 0.23 cm 3 g −1, respectively. The texture and morphology of Z-

USY were further investigated through electron microscopy ( Fig.2). The SEM images of the zeolite reveals characteristic morphology of FAU crystals. Mesopores can be identified through the presence of cavities on the surface of the crystals. These mesopores are dis- tributed randomly within and throughout the zeolite crystals as can be inferred from the TEM images ( Fig. 2d). Typical diameter of steamed mesopores from 20 to 30 nm can be observed together with smaller mesopores of 2–3 nm, as expected from the steam- ing process. From the TEM images achieved from ultramicrotomed samples the presence of the smaller mesopores (ca 2 nm) can fur- ther be observed ( Fig.2f).

Z-USY ST was achieved by surfactant-templating of Z-USY [20] .

This process allows for the development of intracrystalline meso- porosity of tailored size distribution. Indeed, the surfactant tem- plated USY (Z-USY ST) presents a nitrogen physisorption isotherm

that results as a combination of type I and IV, characteristic for hierarchical zeolites featuring a homogeneous size distribution of mesopores ( Fig.1a). The steep nitrogen uptake at the relative pres- sures of 0.3 to 0.4 is characteristic for a tailored mesopore size dis- tribution centered at approximately 4 nm ( Fig. 1b). A lower mi- croporous volume compared to the pristine USY was calculated (0.16 cm 3 g −1), which was ascribed as consequence of the de-

struction of microporosity during the surfactant-templating pro- cess [20] .

The SEM images reveal smooth crystal surfaces for Z-USY ST

( Fig. 3b). The absence of steamed mesopores was further ob- served from the TEM images ( Fig.3d). These latter likewise allow to deduce de presence of intracrystalline mesopores of ca 4 nm. The removal of the steamed mesoporosity during the surfactant- templating process was explained through a crystal expansion the- ory [21] . Alternatively, Ivanova etal., [22] proposed that the dis- appearance of the steamed mesoporosity would result from the formation of a MCM-41-like phase that fills-up the steamed pores during the surfactant-templating process.

X-ray diffraction allows for achieving information from both the position and the full width at half maximum (FWHM) of the peaks ( Fig. 6). From the first, the lattice constant (a 0) of the materials

can be calculated ( Table2). For both Z-USY and Z-USY STa lattice

constant of 24.28 ˚A was calculated indicating identical unit cell size and hence same framework Si/Al ratio. Indeed the Si/Al ratio

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T. Aumond, J. Rousseau, Y. Pouilloux et al. Carbon Trends 2 (2020) 10 0 014

Fig. 2. Electron microscopy images of Z-USY. SEM images presenting the morphology of the crystals and steamed mesopores on the external surface (a, b, c). TEM image of a crystal (d) presenting the steamed mesopores within the crystal (20-30nm). TEM images of ultramicrotomed sample (e, f) evidencing smaller intracrystalline mesopores (ca 2 nm). The insert in e) presents the electron diffraction on that section.

Fig. 3. Electron microscopy images of Z-USY ST . The SEM images (a, b) present smooth crystal surfaces. TEM image of an entire crystal (c) presenting absence of larger mesopores. TEM images on ultramicrotomed sample (d, e, f) showing homogeneous distribution of intracrystalline mesopores. The insert in e) presents the electron diffraction on that section.

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Fig. 4. Nitrogen adsorption and desorption isotherms at 77 K of a) Z-USY (black full symbols) and H-USY (blue empty symbols) and b) Z-USY ST (black full symbols) and H-USY ST (blue empty symbols).

Table 1

Textural parameters of template zeolites (Z-USY and Z-USY ST ) and of ZTCs (C-USY and C-USY ST ).

Sample V micro (cm 3 g −1 ) V meso (cm 3 g −1 ) S BET (m 2 g −1 )

Z-USY 0.29 0.23 887

C-USY 0.73 0.24 1930

Z-USYST 0.16 0.52 918

C-USYST 0.74 0.85 2140

inferred from ICP-OES amount to 16 for both samples. Using the FWHM the coherent crystal size (CCS) and the lattice strain were calculated. As far as the CCS is concerned it is slightly reduced for the Z-USY ST indicting that the development of intracrystalline

mesopores yields smaller diffraction domains within the crystals ( Table1).

3.2. Hybridzeolite/carbonmaterials

As a result of the CVD process the hybrid materials H-USY and H-USY ST were achieved. By comparing the nitrogen physisorption

isotherms of the hybrid materials with the zeolite templates it was determine that the entire microporous volume was filled-up during the CVD process, indicating the completion of the ZTC synthesis. The shape of the isotherms of the H-USY recalls strongly that of the Z-USY, indicating similar mesopore sizes ( Fig.4). As far as H- USY STis concerned the capillary condensation regime has widened

during ZTC synthesis indicating that the mesopore size distribution is larger. This latter observation suggests a rearrangement of the mesoporosity. It is important to note that despite the filling of the micropores the mesoporosity remains accessible, which indicates that carbon deposition only occurred in the micropores.

The amount of carbon within H-USY and H-USY ST was deter-

mined by TGA and amounts to 27 and 15 wt%, respectively. The lower weight loss for the H-USY STcan be explained by the reduced

micropore volume in that sample. Indeed, the microporous volume fraction Z-USY/Z-USY STcompares very well to the mass loss frac-

tion H-USY/H-USY ST and is 1.81 and 1.80, respectively. It is more-

over to note that the shape of the TGA curve is identical for both H-USY and H-USY STindicating that the composition of the ZTC is

very similar ( Fig.5).

By comparing the X-ray diffractograms of the zeolite templates with the hybrid materials important conclusions can be drawn ( Fig. 6 and S1). The lattice parameter (a 0) increases during ZTC

formation from 24.28 ˚A for Z-USY to 24.35 ˚A for H-USY ( Table2). This increase of a 0 might be explained by the distortion of the ze-

Table 2

Structural parameters of zeolite templates (Z-USY and Z-USY ST ) and hybrid materials (H-USY and H- USY ST ) derived from X-ray diffractograms.

Sample a 0 ( ˚A) CCS (nm) ε(x 10 −4 )

Z-USY 24.28 137 2.91

H-USY 24.35 124 3.32

Z-USYST 24.28 127 3.12

H-USYST 24.29 104 3.78

a 0 : lattice parameter, CCS: coherent crystal size,

ε: lattice strain

olite framework during ZTC formation. Indeed, such framework de- formation was observed for the deposition of carbonaceous mate- rial within zeolite channels and intersections [23] . It is to note that in a control experiment in which the zeolites were heated to fist 690 °C and then 790 °C in the absence of ethylene no modification of the position of the XRD peaks was observed, indicating that the deformation of the zeolite structure is related to the development of the ZTC. The lattice parameter increases to a less extent for H- USY ST, which is probably compensated by a higher increase in the

lattice strain for this sample ( Table 2). For both, USY and USY ST

the CCS decreases while lattice strain increases during the ZTC for- mation. The development of the ZTC structure within the zeolite voids hence produces a stress field on the lattice. The reduction of the CCS, which is stronger for H-USY ST, indicates the rearrange-

ment of the mesoporosity, which reduces the size of the crystal- lite domains. Surfactant-templated mesopores in zeolites have in- deed been described to present limited thermal stability and their coalescence has been evidenced for temperatures already below 800 °C [24] .

3.3. Hierarchicalzeolitetemplatedcarbons

After dissolution of the zeolite the pure ZTCs (C-USY and C-USY ST) were obtained. C-USY presents a nitrogen physisorption

isotherms, which shape strongly recalls the one of the zeolite tem- plate ( Fig.7a). Yet, overall nitrogen uptake is almost twice as high compared to the zeolite (0.97 cm 3 g −1). Using the t-plot method

(employing a non-porous graphene sample for recording the refer- ence isotherm, Table S2) a microporous volume of 0.73 cm 3 g −1

was calculated. The mesoporous volume of C-USY amounts to 0.24 cm 3 g −1, which compares well to the value observed for the ze-

olite template ( Table 1), indicating that the overall amount of mesopores is very similar in the ZTC and in the zeolite template. This latter finding strongly indicates that all of the mesoporos-

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T. Aumond, J. Rousseau, Y. Pouilloux et al. Carbon Trends 2 (2020) 10 0 014

Fig. 5. TGA mass loss curves (a) and temperature mass loss distributions (b) of H-USY (black) and H-USY ST (blue).

Fig. 6. X-ray diffractogram of a) Z-USY (black) and H-USY (blue) and of b) Z-USY ST (black) and H-USY ST (blue).

Fig. 7. a) Nitrogen adsorption and desorption isotherms at 77 K and b) DFT pore size distribution for C-USY (black full symbols) and C-USY ST (blue empty symbols).

ity present in the template zeolite was transcribed to C-USY. The DFT pore size distribution ( Fig. 7b) reveals a sharp peak in the micropore region (centered at 1.2 nm) and some contribution of small mesopores (ca. 2 nm), which are approximately of similar size compared to the small mesopores present in USY. Additionally, the increase in nitrogen uptake at high relative pressures ( >0.8) in- dicates the presence of larger mesopores, as in the Z-USY template. From the SEM images of C-USY ( Fig. 8a) it is possible to ob- serve that the morphology and the size of the achieved carbon particles is identical to those of the zeolite template. By studying the surface of the particles more in detail electron clear spots can be distinguished, which can be ascribed to the channel entrance of the larger mesopores ( Fig. 8b). Additionally, channel-like cav- ities can be identified on the particle surface, which feature the

characteristic patterns of steamed mesopores in the template ze- olite ( Fig.8c). TEM images reveal that these larger mesopores are distributed within the entire ZTC particles and are of comparable size (20–30 nm) as was the case for the zeolite template ( Fig.8d). TEM images on ultramicrotomed sample allow to observe the ho- mogeneous distribution of steamed mesoporosity ( Fig. 8e,f). The observation of smaller mesopores is difficult due to the presence of the carbon resin used for the inclusions prior to ultramicro- tomy. Long range order of the micropores can further be deduced from the TEM images ( Fig. 8g,h). The latter is confirmed by the observation of two spots in the electron diffraction (insert Fig.8e). The electronic microscopy images further reveal the absence of non-templating carbon, which would result in the development of graphitic structures on the external surface.

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Fig. 8. Electron microscopy images of C-USY. SEM (a, b, c) and TEM (d) images show the same morphological and textural features as the zeolite template Z-USY. TEM images on ultramicrotomed sample (e, f, g, h) allow to observe the presence of mesopores throughout the sample as in the zeolite template Z-USY. Homogeneous distribution of tailored micropores can further be observed (g, h). The insert in g) presents the electron diffraction on that section. It should be noted that due to the inclusion into a carbon based resin prior to ultramicrotome the sample the observation of small mesopores is extremely difficult.

As far as C-USY ST is concerned the important nitrogen uptake

in the entire relative pressure range of the isotherm indicates the presence of a large distribution of mesopore sizes ( Fig. 7a). This finding suggests the rearrangement of the mesopores dur- ing the ZTC formation. Indeed, a mesopore size distribution form 2–9 nm is derived, which presents a maximum at approximately 3 nm ( Fig. 7b). The mesoporous volume of the C-USY ST amounts

to 0.85 cm 3 g −1, which is a comparable value to what has been

described for the highly mesoporous carbonaceous CMK3 mate- rial [25] . The microporous volume of C-USY ST has been calculated

to account to 0.74 cm 3 g −1, which interestingly is very similar to

the microporous volume achieved for C-USY ( Table 1). This find- ing strongly indicates that the reduced microporous volume in the Z-USY ST template might be apparent, which can easily be ex-

plained by the co-existence of a purely mesoporous phase in the zeolite template resulting from the surfactant-templating process. Ivanova etal. [22] indeed suggested the possibility of partial amor- phization during the surfactant-templating process that would fill- up the steamed mesoporosity of USY. The increased nitrogen up- take at high relative pressures further indicates the presence of larger mesopores ( > 20 nm). The electron microscopy images of C-USY ST confirm what has been deduced from the nitrogen ph-

ysisorption data and reveal the presence of two types of mesopores ( Fig. 9). Conspicuous features of steamed mesopores and of coa- lesced surfactant-templated mesopores can be deduced. Taking all of these observation into account the only explanation for the ab- sence of the steamed mesoporosity in Z-USY ST is the up-filing of

this porosity by an amorphous silica phase during the surfactant- templating process, which is removed together with the zeolite during the HF washing after ZTC formation.

The nitrogen physisorption isotherm of C-USY ST further

presents a H2 hysteresis loop with a sharp desorption step at a relative pressure of 0.42, characteristic for cavitation and hence in- dicating the presence of occulted mesopores [26] . The rearrange- ment of surfactant-templated mesopores during the ZTC synthesis thus leads to a fraction of mesopores that are only accessible via the micropores. From the TEM images of ultramicrotomed sample these occulted mesopores can be observed ( Fig.9e,f).

The XRD patterns of the two hierarchical ZTCs are very simi- lar and present a large peak centered at 6.80 and 6.35 °2theta for C-USY and C-USY ST, respectively ( Fig. 10). For non-crystalline ma-

terials the appearance of diffraction peaks reveals the long-range structural order of the material. The apparition of such a large peak is characteristic for the presence of features presenting simi- lar pore-pore distance [27] . Hence, a micropore-micropore distance of 1.30 and 1.39 nm were calculated for C-USY and C-USY ST, re-

spectively. Taking into account that the micropore size distribution is identical for both samples ( i.e. 1.2 nm, Fig.7b), this results in an average carbon skeleton thickness of 1.2 and 2.1 nm for C-USY and C-USY ST, respectively. This result might be explained by the pres-

ence of a higher degree of intracrystalline mesoporosity in C-USY ST

and hence to a higher number of micropore mouths. These latter seem to be able to accommodate a higher degree of aromatic units, leading to an average larger carbon skeleton.

The XRD patterns further confirm the absence of non-templated carbon species as these would typically result in the development of a large peak between 20–25 °2theta ( Fig. 10). The absence of this feature is an additional indication that all of the carbon in the hierarchical ZTC samples result from the templating of the micro- porosity of the zeolite templates.

The similar chemical composition of the two hierarchical ZTCs (as previously deduced from TGA) was confirmed by comparing the Raman spectra of C-USY and C-USY ST ( Fig. 11). These present

the typical shape expected for ZTCs [28] . The G- and D- bands are observed at 1599 and 1362 cm −1, respectively. The first cor- responds to the stretching vibration of aromatic sp 2 carbon pairs

with E 2g symmetry. Its apparition at higher wavenumbers (com-

pared to graphite, 1580 cm −1) indicates the nano-sized nature of the carbon skeleton of the ZTC [29] . The D-band, characteristic for the lattice vibration with A 1g symmetry is related to the presence

of defects in a graphitic structure. The ratio of the intensities of the D- and GD-band (I D/I G) amounts to 0.77 for both C-USY and

C-USY ST, confirming the very similar chemical nature of the sam-

ples. The decomposition of the spectra further allows to reveal the presence of three additional contributions, centered at 1191, 1282 and 1490 cm −1 ( Fig. S3). The first of these is related to sp 2-sp 3

(9)

T. Aumond, J. Rousseau, Y. Pouilloux et al. Carbon Trends 2 (2020) 10 0 014

Fig. 9. Electron microscopy images of C-USY ST . The SEM images (a, b, c) present the morphology of the template zeolite. Yet, the presence of large mesopores can be deduced on the particle surface. The distribution of mesopores is homogeneous throughout the C-USY ST particles. The TEM images on ultramicrotomed samples (e, f) allow to observe internal (occulted) mesopores. Homogeneous distribution of tailored micropores intercepted by mesopores can be observed in g and h.

Fig. 10. X-ray diffractogram of C-USY (black) and C-USY ST (blue). The insert presents the magnification of the diffractogram in the range 5 to 9 °2theta.

Fig. 11. Raman spectra of C-USY (a) and C-USY ST (b).

bonding and is typically observed in carbons featuring a high sp 3

content [30] . The band at 1490 cm −1has been assigned to intersti- tial defects and typically ascribed to amorphous carbons [31] . The

Scheme 1. Schematic representation of the synthesis of ZTCs. 8

(10)

band at 1282 cm −1 may indicate the presence of fourfold coordi- nated bonds [32] .

4. Conclusion

Zeolite templated carbons (ZTCs) featuring hierarchical poros- ity were achieved through the use of mesoporous zeolites as tem- plates. Steamed mesoporosity from USY was meticulously tran- scribed to the ZTC. The presence of surfactant-templated meso- porosity in the zeolite template allowed for achieving a ZTC pre- senting very high mesoporous volume (comparable to CMK3), whilst simultaneously featuring a microporous volume several times higher than that of classically activated carbons. Hierarchi- cal porosity is of fundamental importance in zeolite science and the development of hierarchization strategies is one of the ma- jor fields of zeolite research. Likewise in ZTCs, the diffusion path length plays an important role in domains where these carbon ma- terials with defined microporous topologies are rated as extremely promising ( e.g. energy conversion or storage). The use of hierar- chical zeolites is one possibility allowing for achieving hierarchi- cal ZTCs. The quality of the transcription of the textural proper- ties seems to depend importantly of the mesoporous geometry and size. Further studies will assess if there is an optimal mesopore ratio and geometry that would allow for neatest transcription of the textural properties. Moreover, as in zeolite science, top down- strategies are likely to be developed to introduce mesoporosity in ZTCs through post-synthetic approaches. Such destructive ap- proaches could rely on specific sp 3 carbon-bond cleaving agents.

DeclarationofCompetingInterest

The authors declare that they have no known competing finan- cial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors acknowledge financial support from the European Union (ERDF) and “Région Nouvelle Aquitaine”.

Supplementarymaterials

Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.cartre.2020.10 0 014 .

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Figure

Fig.  1. a) Nitrogen  adsorption and desorption isotherms at 77  K  and b)  DFT pore size distribution  for Z-USY (black  full symbols) and Z-USY  ST (blue empty symbols)
Fig. 3. Electron microscopy images of Z-USY  ST . The SEM images (a, b) present smooth crystal surfaces
Fig. 4. Nitrogen adsorption and desorption isotherms at 77 K of a) Z-USY (black full symbols) and H-USY (blue empty symbols) and b) Z-USY  ST (black full symbols) and  H-USY  ST (blue empty symbols)
Fig. 7. a)  Nitrogen adsorption and desorption isotherms  at 77 K and b) DFT  pore size distribution  for  C-USY (black  full symbols)  and  C-USY  ST (blue  empty  symbols)
+3

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