# First Acidic Macro-Mesocellular Aluminosilicate Monolithic foams "SiAl(HIPE)" and their Catalytic Properties

Damien P. Debecker, Cédric Boissière, Guillaume Laurent, Stéphanie Huet, Philippe Eliaers, Clément Sanchez, Rénal Backov

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## To cite this version:

Damien P. Debecker, Cédric Boissière, Guillaume Laurent, Stéphanie Huet, Philippe Eliaers, et al. First Acidic Macro-Mesocellular Aluminosilicate Monolithic foams "SiAl(HIPE)" and their Catalytic Properties. Chemical Communications, 2015, 51 (74), pp.14018-14021. 10.1039/C5CC05328E. hal-01191422v2

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Chemical Communications

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COMMUNICATION

Damien P. Debecker, Cédric Boissière et al.

First acidic macro-mesocellular aluminosilicate monolithic foams

"SiAl(HIPE)" and their catalytic properties
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Cite this: *Chem. Commun.*, 2015, **51**, 14018

Received 29th June 2015,
Accepted 29th July 2015

DOI: 10.1039/c5cc05328e

www.rsc.org/chemcomm

# First acidic macro-mesocellular aluminosilicate monolithic foams "SiAl(HIPE)" and their catalytic properties†

Damien P. Debecker,*a Cédric Boissière,*b Guillaume Laurent,b Stéphanie Huet,c
Philippe Eliaers,d Clément Sanchezb and Rénal Backovc

A new type of acidic macrocellular and mesoporous silica–alumina foam is obtained *via* a one pot alkaline sol–gel route coupled with a concentrated emulsion-based templating technique. The mixed oxide monolith exhibits high surface acidity, translating into excellent performance in the acid-catalyzed dehydration of bioethanol to ethene.

Aluminosilicates are of utmost importance as catalysts and
catalyst supports.¹ Amorphous aluminosilicates are used as
mild acidic supports for various active species such as, for instance,
Mo, W or Re oxides for olefin metathesis;² Ni, W, Co and Mo oxides
for hydrodesulfurization reactions;³ Ni for reduction reactions;⁴ Pd
and Pt for hydrogenation reactions,⁵ *etc*. They are also employed as
catalysts themselves in several petrochemical processes, including
cracking.⁶ Their acidity can be somewhat tuned to meet the specific
requirement of a particular reaction.

Zeolites are synthetic microporous crystalline aluminosilicates,
in which Al atoms replace Si atoms in the framework, thereby
generating uncompensated charge and strong acidity.¹ᵃ Even if
their composition, acidity and structure can be finely tuned,
zeolites usually exhibit only small pores in which the diffusion
of reactants and products is hampered. Significant steps forward
have been made in the design of truly mesoporous acidic
aluminosilicates when templating agents have been used to
generate mesopores in zeolitic materials.⁷ Further developments
in this field are still flourishing, taking benefits of advanced
polymer science or original processing techniques.⁸

Nevertheless, whether amorphous or crystalline, micro- or
mesoporous, catalytic materials in the form of powders have to
be shaped as extrudates, pellets or monoliths before being used
in industrial flow processes.⁹ This step is usually not straight-
forward, as (i) appropriate additives must be found that ensure
physical strength without compromising catalytic performance,
(ii) diffusion limitation issues must be addressed and (iii)
attrition must be avoided.¹⁰ An alternative to shaping powders
is to impregnate structured supports with active components.¹¹
There also, potential technical issues are numerous: lack of
adherence of the catalytic powder to the walls of the structured
support, pore plugging, drastic decrease of the specific loading
and so forth.

One-step preparation of structured materials with the required properties is hence highly desirable. In this context, the integration of sol-gel chemistry and physical chemistry of complex fluids has led to the genesis of various advanced functional materials.¹² The one-step preparation of macrocellular silica monoliths is a noteworthy example.¹³ Taking advantage of sol-gel chemistry coupled with an emulsion technique, macroscopic silica blocks of defined shape can be obtained, featuring high surface area and good mechanical strength (5–7 MPa Young’s modulus).¹⁴ The preparation is based on the ‘High Internal Phase Emulsion’ (HIPE) method.¹⁵ Si(HIPE) can be prepared as self-standing macroscopic objects that can be easily manipulated. Along with other diverse applications, these materials when hybridized¹⁵ proved to be successful as catalyst supports for lipases used in oleochemistry,¹⁶ for Pd nanoparticles used in C–C coupling,¹⁷ *etc*. Such materials can be cast directly in tubular reactors and be employed as catalyst supports for flow reactions even with bulky molecules.¹⁸

The incorporation of alumina into such a system would pave
the way to structured acid catalysts. However, this is not
straightforward since chemical synthesis conditions used for
the preparation of such materials are systematically based on
colloidal or acidic silica chemistry. The latter routes are totally
inefficient for the true incorporation of aluminum cationic
species into the silica network.¹⁹ While Al in tetrahedral

<footnote>ᵃInstitute of Condensed Matter and Nanoscience - Molecules, Solids and reactivity
(IMCN/MOST), Université catholique de Louvain, Croix du Sud 2/17,
1348 Louvain-La-Neuve, Belgium. E-mail: damien.debecker@uclouvain.be</footnote>

<footnote>ᵇSorbonne Universités, UPMC Univ Paris 06, CNRS, Collège de France,
Laboratoire de Chimie de la Matière Condensée de Paris (LCMCP),
11 place Marcelin Berthelot, 75005, Paris, France. E-mail: cedric.boissiere@upmc.fr</footnote>

<footnote>ᶜCentre de Recherche Paul Pascal, UPR8641, Université de Bordeaux 1,
115 Avenue Albert Schweitzer, 33600 Pessac, France</footnote>

<footnote>ᵈCertech (Centre de Ressources Technologiques en Chimie), rue Jules Bordet,
7180 Seneffe, Belgium</footnote>

<footnote>† Electronic supplementary information (ESI) available: Full experimental details and additional data. See DOI: 10.1039/c5cc05328e</footnote>
<figure><img src="image_6.png" /><figcaption>Fig. 1 (left) Picture of a SiAl(HIPE) monolith, (right) SEM micrograph of the SiAl(HIPE).</figcaption></figure>

coordination is required to obtain strongly acid heterogeneous catalysts,²⁰ classical routes usually generate aluminum oxide chunks in an octahedral environment.

In the present study the emulsion-based sol-gel route is modified in an alkaline version to meet the ambitious agenda of obtaining, in one-step, a structured monolithic acidic catalyst bearing hierarchical porosity. The adapted protocol is described in detail in the ESL† Briefly, TEOS is incorporated into the emulsion oil phase (dodecane) along with the TTAB surfactant. Aluminum isopropoxide is added in the water phase along with TPAOH. A stable emulsion is formed after vigorous stirring and is simply allowed to stand for 1 week, washed and calcined. In this way, SiAl(HIPE) was prepared in the form of cm-scale solid monoliths (Fig. 1 and ESI†). Macroscopically, SiAl(HIPE) materials appear similar to Si(HIPE).

Pieces of monolith were scratched and observed using scanning electron microscopy (SEM). This revealed the macrocellular structure of the materials (Fig. 1). Interconnected macro-cells have a diameter in the 5–45 µm range. As compared to the well-described Si(HIPE) materials, these macrocells are smaller and present a narrower pore size distribution. This can tentatively be associated with the fact that the SiAl(HIPE) monoliths have been obtained in alkaline pH. Under these conditions, condensation reactions are expected to proceed at a faster rate, generating high shearing forces earlier in the emulsion process, thus minimizing the oil droplet diameters and favoring size monodispersity.¹⁴ The atomic Si/Al ratio was estimated over several wide zones of different monolith pieces using the EDX probe mounted on the SEM apparatus to be 9.7 ± 0.2, not very far from the nominal value of 12. Possibly, a portion of the TEOS did not react at the oil/water interface and was eliminated during the washing. The composition was shown to be fairly homogeneous throughout the sample (ESI†). The sample is macroporous as attested using SEM and confirmed by Hg-porosimetry (Fig. 2). SiAl(HIPE) is characterized by an intrusion volume of 10.2 cm³ g⁻¹ and a porosity of 89%. Its bulk density is 0.087 g cm⁻³. In terms of macrostructures, the alkaline synthesis method allows the production of materials that are as good as those previously reported Si(HIPE). However, the skeletal density obtained is only 0.80 g cm⁻³, which is lower than Si(HIPE) (~1.50 g cm⁻³). Macrocells are interconnected by relatively monodisperse pore openings around 4 µm in size.

N₂-physisorption measurements demonstrate that SiAl(HIPE) is mesoporous. The BET surface area reaches 900 m² g⁻¹. Contrary to the Si(HIPE) series, a t-plot analysis of the isotherm proves that no

<figure><img src="image_7.png" /><figcaption>Fig. 2 (left) Size distribution of the macroscopic pore opening obtained by mercury intrusion porosimetry and (right) N₂-sorption isotherms.</figcaption></figure>

significant microporosity is present in this material, the nitrogen uptake at very low relative pressure being attributed to the high surface area of the samples. A well-defined hysteresis loop in the isotherms translates into a sharp pore size distribution obtained from the adsorption branch, centered at ca. 3.6 nm via the BJH method, and 4.0 nm via the Broekhoff and De Boer method (ESI†). The presence of a so-called catastrophic desorption at P/P₀ ~ 0.42 make the estimation of the mesopores restriction size impossible.

The sample acidity was measured using NH₃-chemisorption and compared to both a popular commercial aluminosilicate support (Aldrich, Grade 135, 475 m² g⁻¹, 12 wt% Al₂O₃)²² and Si(HIPE)¹⁷ᵇ (Fig. 3). The method allows a quantitative analysis of acid sites, with a distinction between the weak and the strong sites.²¹ As expected, Si(HIPE) is virtually not acidic. Aluminosilicate samples on the other hand retain NH₃ in substantial quantities and SiAl(HIPE) exhibited a total number of acid sites per gram of a sample significantly higher than the reference commercial aluminosilicate. The amount of strong acidic sites is almost 3-fold as compared to that of the reference. If the two aluminosilicate materials are compared on a surface basis, the reference exhibits more acidic sites. However, SiAl(HIPE) still exhibits more strong acid sites per surface unit.

NMR experiments were conducted to gain insights into the environment of Si and Al atoms in SiAl(HIPE). It appears important, indeed, to verify that the sol-gel process performed in basic pH did not lead to the stabilization of alumina domains dispersed in a silica matrix, but that a truly mixed oxide was formed instead. ²⁹Si MAS NMR spectra (Fig. 4) show broader peaks strongly shifted to the left, which is typical for aluminosilicates.²³ This shows that Si and Al precursors have reacted together during the preparation to form Si–O–Al bonds.

<figure><img src="image_8.png" /><figcaption>Fig. 3 Acidity of SiAl(HIPE) measured using NH₃-chemisorption²¹ and compared to the commercial reference and to Si(HIPE). The acidity is expressed as the amount of NH₃ chemisorbed per gram of catalyst (left) or per m² of catalyst (right).</figcaption></figure>

<figure><img src="image_9.png" /></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 4 (up) $^{29}$Si MAS-NMR spectra of Si(HIPE) and SiAl(HIPE) and (bottom) $^{27}$Al MAS-NMR spectrum of SiAl(HIPE).</figcaption></figure>

The $^{27}$Al MAS-NMR spectrum obtained for SiAl(HIPE) shows a large proportion of tetrahedrally coordinated framework Al atoms (signal at ca. 56 ppm, abbreviated Al$^{IV}$). The latter is typically formed in amorphous aluminosilicates, along with pentacoordinated Al (broad signal around 30 ppm, abbreviated Al$^{V}$) and octahedral Al (ca. 2 ppm, abbreviated Al$^{VI}$).$^{2a,24}$ In SiAl(HIPE), however, the proportion of Al$^{IV}$ species versus Al$^{V}$ and Al$^{VI}$ is significantly higher than what is typically found in the reference aluminosilicate.$^{25}$

Such a high content of Al$^{IV}$ species is characteristic of pre-
zeolitic amorphous aluminosilicate materials prepared in basic
media under similar conditions.$^{19}$ As a consequence, one may
explain both the sol–gel condensation and the good incorporation
of aluminum centers within the silica network by the progressive
dodecane/water interfacial hydrolysis of TEOS and deprotonation
of silicic acid that locally induces a pH drop (Fig. 5), allowing the
polycondensation of fleeting aluminum hydroxide with silicate

<figure><img src="image_11.png" /><figcaption>Fig. 5 Diffusion-limited sol–gel aluminosilicate co-condensation mechanism taking place at the dodecane/water interface.</figcaption></figure>

species. Both TEOS and TTAB diffusion within the dodecane phase
and aluminate diffusion within the aqueous phase being slow,
there is time for a progressive self-assembly of the positively
charged TTAB structuring agent and negatively charged alumino-
silicate oligomers, resulting in the mesostructured SiAl(HIPE).

An interesting feature of solid state NMR spectroscopy is the
possibility of checking spatial proximities between nuclei. For
quadrupolar nuclei like $^{27}$Al, the TRAPDOR experiment can be
used.$^{27}$ This has been undertaken on a dehydrated sample to
discriminate which protons are neighboring $^{27}$Al centers and
thus which ones are expected to be implicated in the catalytic
activity. The experiment consists of comparing two spectra, a
reference one without $^{27}$Al irradiation and an irradiated one, at
increasing echo times. The closer the involved spins, the higher
will be the dephasing effect. A strong effect is observed around
5 ppm, corresponding to Si–(OH)–Al hydrogen-bonded species;
an intermediate effect is seen at 2–2.5 ppm, attributed to Si–OH
close to the Al center; and no effect is denoted for isolated
Si–OH at 1.8 ppm (Fig. 6). Thus, the strong Si–O–Al Brönsted
acidity observed here matches the high standard of some
already reported powdery materials produced by precipitation
or aerosol processing, like those obtained from zeolite seeds or
amorphous prezeolitic materials.$^{19}$

From catalytic point of view, both the SiAl(HIPE) and the
reference materials catalyze the dehydration of ethanol with
high selectivity (~95%) under the conditions used here (Fig. 7).
For SiAl(HIPE), conversion remains almost complete up to a
space velocity (WHSV) of 23.8 h⁻¹. Note that longer contact
times are classically reported in the literature.²⁸ Under our
conditions and with a WHSV of 23.8 h⁻¹, the specific productivity
reaches 13.3 g<sub>ethene</sub> g<sub>cat</sub>⁻¹ h⁻¹ for SiAl(HIPE), while the commercial
sample produces only 10.4 g<sub>ethene</sub> g<sub>cat</sub>⁻¹ h⁻¹. As the dehydration of
ethanol is known to occur on acid sites, such high ethene
productivity is related to the enhanced acidic properties of the
SiAl(HIPE). By-products include butenes, acetaldehyde and diethyl
ether. The full product selectivity profile is provided in the ESI.†
Acetaldehyde selectivity systematically remained below 0.5%.
While diethyl ether is obtained via a less complete dehydration
of ethanol, 1-butene is the product of a consecutive dimerization
reaction of ethylene. Thus, as expected, selectivity for diethyl ether
was lower and 1-butene selectivity was higher for SiAl(HIPE) as

<figure><img src="image_12.png" /><figcaption>Fig. 6 Echo MAS $^{1}$H NMR spectra (sample dehydrated at 200 °C). The chemical shift is based on the ($^{27}$Al)-$^{1}$H TRAPDOR experiment (spectra were simulated using dmfit program;$^{26}$ see the ESI$^{†}$)</figcaption></figure>

<figure><img src="image_13.png" /></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 7 Conversion of ethanol at 400 °C with (■) SiAl(HIPE) and (▲) the reference. 1.2 g of the catalyst. Selectivity to ethanol is always in the range of 94.2–97.6% (see Table S1, ESI,† for a full product profile). Feed is first generation bioethanol from BioWanze; 99.7 wt% ethanol and –0.3 wt% water evaporated in a preheating zone.</figcaption></figure>

compared to the commercial reference. This again points to a higher acidity and thus a higher activity of the SiAl(HIPE).

Combining sol–gel chemistry and physical chemistry of complex fluids, the new route presented here allows the preparation of meso–macrocellular self-standing objects that are easy to cast and manipulate and that exhibit open and interconnected macroporosity, high surface area and strong acidic properties. The textural and structural features of these new aluminosilicate monoliths make them particularly attractive for flow processes. The formation of a true mixed aluminosilicate generates abundant and strong surface acid sites, which makes these materials highly attractive for acid-catalyzed reactions. Effectiveness of these catalysts is demonstrated here in the case of bioethanol dehydration, and good performance can be expected in other processes involving bulkier molecules like cracking, isomerization, and alkylation reactions.

The French Région Ile de France SESAME program is acknowledged for financial support (700 MHz spectrometer).

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