Synthesis of Highly Accessible Gallosilicates via Impregnation Procedure: Enhanced Catalytic Performances in the Conversion of Glycerol into Solketal

The European Society Journal for Catalysis

Supported by

Accepted Article

Title: Synthesis of highly accessible gallosilicates via impregnation

procedure: enhanced catalytic performances in the conversion of

glycerol into solketal

Authors: Hussein Hussein, Alvise Vivian, Luca Fusaro, Michel

Devillers, and Carmela Aprile

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To be cited as: ChemCatChem 10.1002/cctc.202001172

Link to VoR: https://doi.org/10.1002/cctc.202001172

01/2020

10.1002/cctc.202001172

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Synthesis of highly accessible gallosilicates via impregnation

procedure: enhanced catalytic performances in the conversion of

glycerol into solketal

Hussein Hussein,^[a]Alvise Vivian,^[b]Luca Fusaro,^[b]Michel Devillers,*^[a]Carmela Aprile*^[b]

Dedication ((optional))

Abstract: Two mesoporous gallosilicates with extra-small particle

size were synthesized via wet impregnation starting from lactate or

citrate-type Ga(III) precursors. The structural and textural properties

of the materials were extensively characterized by different

techniques. Both materials consist of nanosized particles and display

a remarkably high surface area and a pore size distribution typical of

mesoporous MCM-41-like materials. The two solids were tested as

catalysts in acetalization of glycerol with acetone to produce solketal

(2,2-dimethyl-1,3-dioxolane-4-methanol). Both catalysts displayed

excellent performances and achieved higher turnover numbers

compared to other Ga-silicates reported in literature. The

outstanding activity of these catalysts is attributed to the combination

of acidity, large specific surface area as well as to the full

accessibility of the active sites as consequence of the impregnation

procedure. The best catalysts are truly heterogeneous with absence

of Ga leaching and reusable in successive catalytic cycles after

thermal treatment without significant deterioration of the catalytic

performance.

characterized by a regular array of hexagonal pores, narrow

pore size distribution and high surface area.^[4]Despite these

promising features, the catalytic applications of unmodified all

silica based solids are hindered by the lack of acidity or other

active functionalities. The introduction of heteroatoms as single

sites in the silica architecture (M-MCM-41) has therefore

attracted a growing interest. The reasons of this great deal of

attention stand mainly on the possibility of tuning the Lewis/

Brønsted acid properties of the material in function of the nature

and loading of the selected metal center with subsequent

application in a broad range of catalytic reactions including

alkene alkylation or polymerization, esterification, acetalization

etc.^[5]It was reported that Brønsted solid acid sites are

generated by the hydroxyl functionalities bridging the silicon and

the metal center inserted as single site (-Si(OH)M- with M= Al,

Ga, Fe…)^[6-7]while the Lewis acidity is provided by tetrahedral M

sites with an unsaturated coordination shell as well as by

surface M sites with an extended penta- or hexa-coordination

geometry. The insertion of a metal center in the silica structure is

usually achieved via a co-synthesis strategy starting from silica

and metal precursors in the presence of a sacrificial structure-

directing agent. This approach allows obtaining a homogeneous

dispersion of metal centers in the silica architecture avoiding

phase segregation with formation of SiO₂and metal oxides

aggregates. However, a certain amount of the metal remains

trapped in the pore walls and is, hence, not accessible to the

reactants. An alternative post-synthesis strategy involves the

use of a preformed silica solid support which is functionalized

with the selected catalytic active species. In this case, the metal

precursors are loaded into the porous support usually by wet or

incipient wet impregnation.^[8]Particularly chosen is the wet

impregnation method which has the advantage of both technical

simplicity and reproducible metal loading.^[9-10]In this method, the

volume of the precursors-containing solution exceeds the total

pore volume of the material resulting in the deposition of a large

amount of precursor at the external surface of the solid. The

advantage of this approach is the absence of intra-wall metal

center, hence a large availability of surface catalytic active sites.

However, the simulation of a surface single site is more difficult

and the possible sintering with the formation of metal oxides

may constitute a drawback limiting the catalytic performances of

the solids. Hence, the selection of the appropriate metal

precursor allowing a homogeneous and stable dispersion of

metal centers at the surface of the selected porous silicate

constitute a key pillar and a challenge in the synthesis of a

catalyst via post-functionalization.

Introduction

The development of environmentally compatible and recyclable

heterogeneous catalysts with improved performances in the

production of fine chemicals is a field in continuous evolution

attracting a constant interest at both academic and industrial

levels. The design of active catalysts involves different strategies

going from the identification of novel catalytically active species

to the tailored design of selected structural, textural and

morphological features of the catalytic support.^[1]

Historically, one of the most known and industrially

employed class of solids is constituted by the zeolites. In 1991,

the synthesis of the M41S family has expanded the range of

application of porous materials to mesoporous dimension.^[2-3]Of

particular interest is the siliceous MCM-41 which is

[a]

H. Hussein, Prof. Dr. M. Devillers

Institute of Condensed Matter and Nanosciences - Université

catholique de Louvain, Place Louis Pasteur, 1 box L4.01.03, 1348

Louvain-la-Neuve, Belgium.

[b]

[*]

Prof. Dr. C. Aprile, Dr. L. Fusaro, Dr. A. Vivian

Unit of Nanomaterials Chemistry, University of Namur, Department

of Chemistry, 5000 Namur, Belgium.

Corresponding authors: michel.devillers@uclouvain.be ;

carmela.aprile@unamur.be.

Herein, two different gallium precursors were synthesized,

fully characterized and employed as source of gallium to

impregnate the surface of a porous silica support with a MCM-

Supporting information for this article is given via a link at the end

of the document.

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41-like structure and small particle size to avoid diffusion

limitations.

The good activity for the three heterogeneous catalysts was

attributed to a large specific surface, a relatively hydrophobic

surface and the accessibility to the acid sites.

The solids were tested as catalysts selecting the valorization

of glycerol as target reaction. Currently, glycerol is produced in

large quantities as a concomitant product in the manufacturing

of biodiesel from vegetable oils. For this reason, suitable uses of

glycerol surpluses were suggested through its condensation with

readily available acetone, to yield solketal (2,2-dimethyl-1,3-

dioxolane-4-methanol),^[11]a compound with high added-value,

according to scheme 1.

The present research work deals, for the first time, with the

catalytic behavior of extra-small particle size (XS, below 100 nm)

of mesoporous MCM-41 gallosilicate materials synthesized via

wet impregnation in the acetalization of glycerol with acetone to

produce solketal under heterogeneous acid-catalyzed conditions.

The influence of various experimental parameters such as

reaction time, catalyst amount, reaction temperature and

acetone/glycerol molar ratio was investigated. These materials

are expected to present an attractive combination of active and

highly accessible sites, leading to improved activity in the title

reaction. The catalytic results were correlated with the physico-

chemical properties of the different catalysts synthesized and

illustrate the high potential of these materials in the context of

sustainable production of valuable chemicals.

OH

O

HO

OH

HO

Ga-silicates

+

Solketal

-

H₂O

O

+

O

Results and Discussion

Two gallium(III) precursors, the neutral gallium lactate

monohydrate Ga(C₃H₅O₃)₃.H₂O (GaLac) and the ammonium

OH

2,2-dimethyl-1,3-dioxane-5-ol

salt

of

an

anionic

dicitratogallate(III)

complex,

(NH₄)₅[Ga(C₆H₄O₇)₂].2H₂O (GaCit), were synthesized and used

as sources of Ga for the preparation of the final catalysts. The

different nature and polarity of the selected precursors may

contribute to the understanding of the role played by the

interaction of the active species with the surface of the support,

which in turn influences the dispersion of the molecules on a

support as well as the catalytic performances of the final solid.

Prior to the synthesis of the gallium-based catalysts, the two

precursors were extensively characterized via different

techniques. Thermogravimetric analysis (TGA) was employed to

evaluate the thermal stability of the complexes. As can be

clearly observed in Figure 1, both gallium (III) complexes

undergo a three-step degradation up to a final decomposition

temperature in the range between 700 and 800°C (Table S1) to

yield β-Ga₂O₃. The presence of gallium oxide particles was

proved via X-ray diffraction (Figure S1).

Scheme 1. Glycerol acetalization with acetone.

Solketal is widely used as a solvent in several large-scale

applications as paint and ink formulation,^[12]in the

pharmaceutical industry,^[13]and as a fuel additive.^[14]The

synthesis of solketal through the condensation of glycerol and

acetone is usually catalyzed by homogeneous Brønsted acid

catalysts such as sulfuric acid or p-toluenesulfonic acid.^[15]

Although the yield of these methods is excellent,^[16]the reagents

are expensive, the procedure is long and such approach showed

disadvantages concerning the catalyst’s recovery, production of

large amount of waste during the separation step and others

concerning the cleaning of the catalyst. Nevertheless, attempts

to improve this approach were carried out by the substitution of

the original homogeneous phase catalyst by an insoluble

heterogeneous one which possesses the known advantages of

easy separation and possibility of recovery. Particularly, acid

functionalized mesoporous silica has great advantages in this

research domain because of its wide accessibility, high porosity,

inert environment and reusability. In 2008, da Silva et al. studied

the acetalization of glycerol, using zeolite Beta with a Si/Al ratio

of 16 as a heterogeneous catalyst without any hazardous

solvent.^[17]They showed that the Brønsted acid sites of the

zeolite leads to a conversion of 95% within the reaction time of

30 minutes. Another series of heterogeneous catalysts has been

designed by Vicente et al. in 2010 based on different sulfonic

acid-modified mesostructured silicas.^[18]They obtained 83%

catalytic conversion of glycerol to solketal within 30 minutes

using arenesulfonic acid-functionalized silica (Ar-SBA-15). Also,

it was reported that glycerol acetalization with acetone was

carried out over mesoporous substituted silica in which

heteroelements were incorporated in the silica framework (Zr-

TUD-1, Hf-TUD-1 and Sn-MCM-41).^[19]The authors showed that

mesoporous Lewis acid catalysts were active for this reaction.

Figure 1. TGA analysis of GaLac and GaCit.

The stoichiometry of the complexes was verified via

combustion chemical analysis and inductively coupled plasma

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optical emission spectrometry (ICP-OES). Both techniques

allowed evidencing an excellent agreement with the nominal

values thus confirming the composition of the synthesized

compounds as well as their purity (see experimental section for

more details). The presence of an interaction between the metal

center and the ligand through formation of Ga-O bonds was

investigated via IR and NMR spectroscopy. The IR spectra of

molecules, the main phase is confirmed to be Ga(C₃H₅O₃)₃.H₂O

(Figure S4). On the other hand, GaCit did not display a well

resolved crystalline pattern. However, it deserves to be

mentioned that neither of the XRD patterns show the presence

of contributions from Ga₂O₃separate phases.

both samples revealed the presence of a strong contribution

1

close to 1600 cm^-

corresponding to the antisymmetrical

stretching frequency of carboxylate groups coordinating the

metal center. This conclusion is supported by literature data. As

reported previously^[20]the coordination of the gallium atom by

carboxylates is evidenced by the broadening of the ʋ_as(COO^-)

together with a shift to lower frequencies from 1700 cm^-1in the

free lactate/citrate ligands to c.a. 1600 cm^-1in the complexes.

The corresponding symmetric stretching ʋ_s(COO^-), appeared at

1455 cm^-1for the lactate and 1405 cm^-1for the citrate complex,

respectively. The additional band observed in the range 550-650

cm^-1for both complexes can be attributed to ʋ(Ga-O) (Figure

S2).

The gallium complexes were also investigated by ¹³C-NMR

spectroscopy in aqueous D₂O solution using DSS as an internal

standard. The solution-state ¹³C NMR spectrum of lactate

complex recorded at pH = 5 showed three main peaks (22 ppm

for CH₃group, 70.1 ppm for CH and carboxylic group close to

183 ppm) which are slightly shifted in comparison with those

recorded for the free lactic acid ligand at the same pH (Figure

S3a). The ¹³C NMR spectrum of the citrate complex recorded at

pH = 8 displayed eight main peaks that were assigned on the

basis of the ¹³C NMR spectrum of free citrate recorded at the

same pH. The citrate peaks were observed at 182.4, 179.7, 75.7

and 46.2, ppm while the four other ones (at 187, 179.7, 74.5 and

46.8 ppm) refer to Ga(III)-bound citrate (Figure S3b).

Figure 2. ⁷¹Ga liquid state NMR spectroscopy of gallium complexes as

function of pH, (a), (b) and (c) Ga(III) lactate at pH = 2, 5 and 9 respectively.

(d), (e) and (f) Ga(III) citrate at pH = 2, 5 and 9 respectively.

The two precursors were employed to prepare a series of

Ga-based structured catalysts via impregnation of the Ga

complexes on a solid support. For this purpose, a nanosized

mesoporous silicate (XS-SiO₂)^[22]was selected as support due to

its favorable features such as high surface area, narrow pore

size distribution, uniform particle size and good thermal stability.

Moreover, it is known that the extra-small particle size (XS) of

this material may improve the accessibility to/from the active

sites thus reducing the diffusion limitations. The surface

functionalization with the gallium precursors was performed via

wet impregnation followed by solvent evaporation. In order to

remove the organic groups and obtain finely dispersed Ga-

Evidence of the formation of coordination complexes may be

obtained by ⁷¹Ga NMR spectroscopy. ⁷¹Ga is a quadrupolar

nucleus (I=3/2) with a relatively large natural abundance (39.8%). based species (simulating a single-site center) at the solid

It is sensitive to both the symmetry of its local environment and

to molecular dynamics in solution. The signals of ⁷¹Ga are sharp

when the metal has a highly symmetrical environment and a fast

molecular tumbling, as in the case of the resonance lines

observed for octahedral [Ga(D₂O)₆]³⁺in acidic solution and

tetrahedral [Ga(OD)₄]^-in alkaline solution. The spectrum of the

most acidic solution (pH = 2) is dominated by a relatively sharp

signal at δ = 0 ppm (Figure 2a,d). Similarly, the spectrum of the

most basic solution (pH = 9) has a sharper signal at δ = 222

ppm (Figure 2c,f). At pH = 5, two broad ⁷¹Ga signals are

observed (∆_1/210 kHz) at δ = 33 ppm and δ = 27 ppm (Figure

2b,e), which are assigned to the gallium lactate and citrate

complexes, respectively, in agreement with the results reported

in literature.^[21]

The X-ray powder diffraction pattern of the GaLac (see

experimental section for more details) revealed the presence of

a series of peaks which were indexed as a monoclinic crystal

system with unit cell parameters a = 9.8152 Å, b = 4.7142 Å, c =

17.5147 Å, β = 100.431°, V = 797.028 Å, possible space group

is P2₁or P2. From the unit cell volume and assuming that the

sample contains only gallium cations, lactate anions and water

surface, a thermal treatment was performed. However, it is

known that a too harsh thermal treatment could favor the

migration of the metal-based species with possible sintering

phenomena. On the other hand, a calcination temperature below

550°C will produce only a partial removal of the organic ligands.

Hence, two different thermal treatments were investigated while

preserving the temperature of 550°C. The first approach

involves a gradual increase of temperature until 550°C under

argon followed by a treatment of 5 h in presence of air (method

A) while the second procedure implies that all the steps were

performed only under air (method B).

For the sake of comparison, two additional solids were

prepared by impregnating the XS-SiO₂material with

a

suspension of pre-formed Ga₂O₃particles (XS-Ga₂O₃-A) and

with a Ga(NO₃)₃solution (XS-GaNit-A). This last procedure was

previously reported in literature^[23]for the generation of finely

dispersed Ga₂O₃species at the surface of zeolites. The

physicochemical features of all the XS-Ga-silicates were studied

by means of

a combination of different characterization

techniques. The complete characterization of the two samples

calcined following the method A (XS-GaLac-A and XS-GaCit-A)

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will be discussed here in detail, while the characterization of the

remaining solids (XS-GaLac-B, XS-GaCit-B, XS-GaNit-A and

XS-Ga₂O₃-A) can be found in the Supporting information (SI).

The amount of metal precursors loaded on XS-SiO₂support

corresponds to a nominal ratio of Si/Ga = 74. However, the

experimental Si/Ga ratios determined by ICP-OES were higher

than expected thus indicating a lower loading of gallium species.

This decrease could be attributed to a partial loss of gallium

precursors as consequence of the thermal treatment, especially

in the case of the citrate complex. This hypothesis was

supported by the ICP analysis performed before calcination

which gave a Si/Ga ratio very close to 74. Moreover, TGA

results revealed that the experimental amount of Ga₂O₃was

lower that the nominal one (Table S1). Combining both results

allows confirming the partial loss of Ga species during the

calcination process.

Transmission electron microscopy demonstrated that all XS-

Ga solid materials consist of nanosized spherical particles with a

typical MCM-41-like ordering (Figure S8).

The infrared data presented in Figure 3 confirm the

incorporation of gallium in the silica framework due to the

presence of ʋ_as(Si–O–Ga) band at 960 cm^-1after the Ga

impregnation on XS-MCM-41. This band was preserved even

after the calcination process.

The characterization of the chemical environment of gallium

via solid state nuclear magnetic resonance represents

a

challenging task. The acquisition of high quality solid state-NMR

spectra was hindered by low percentage of Ga inside the

mesoporous silica structure combined with the intrinsic

drawbacks of ⁷¹Ga such as large chemical shift anisotropy and

quadrupolar interaction.^[24]Aiming at reducing the width of the

signals, ⁷¹Ga Magic angle spinning (MAS) NMR spectra of

calcined samples were recorded at high spinning frequency (25

kHz) (Figure 4). The mesoporous gallosilicate materials XS-

GaLac-A/B, XS-GaCit-A/B and XS-GaNit-A showed a single

intense peak at 150 ppm indicating that gallium is prevalently

present as a single site in tetrahedral coordination. This finding

is in agreement with previous reports that confirmed the

tetrahedral incorporation of gallium in framework at low metal

loading.^[24]In addition, the spectrum of XS-GaNit-A and XS-

Ga₂O₃-A showed a peak at -10 ppm which can be assigned to

the octahedral extra-framework Ga species.

Nitrogen physisorption measurements evidenced a very high

specific surface area (SSA) for all solids (Table 1) and a narrow

pore size distribution centered at 2.6 nm indicating that the

impregnation/calcination procedure had no detrimental effect on

the structured silica (Figure S5). As expected all the samples

display a type IV isotherm typical of mesoporous structured

materials (Figure S6). Moreover, a clear hysteresis loop at

higher relative pressure was observed and ascribed to a second

type of porosity due to disordered interparticle cavities originated

as consequence of the small particle size. A slight decrease of

SSA and pore volume was observed for the impregnated

materials compared to the pristine support. As reported in Table

1, both catalysts prepared employing the citrate complex display

a more significant decrease in SSA compared to the ones

synthesized via impregnation of gallium lactate complexes. It is

important to underline that the citrate complex is negatively

charged and associated with ammonium counter-ions unlike the

neutral gallium lactate. This might influence the interaction of the

citrate complex with the XS-MCM-41 support and slightly hinder

its homogeneous dispersion on the support surface. Hence, the

relatively low specific area could be ascribed to the presence of

a higher fraction of poorly dispersed Ga species (most probably

in the form of oligomers or small clusters) located at the surface

or partially blocking the pores in Ga citrate-based catalysts.

Low angle powder X-ray diffraction performed on all samples

exhibit a broad and intense diffraction peak, which can be

attributed to the hexagonal structure of the mesoporous material

and is assigned to the d₁₀₀diffraction (Figure S7a); this provides

evidence that the mesoporous structure is preserved after

impregnation with the gallium species and subsequent

calcination at 550°C. More importantly, the diffraction pattern

recorded in the 2θ range comprised between 5° and 80° did not

show any evidence of the formation of Ga₂O₃species thus

suggesting a good dispersion of the metal precursors on the

silica surface (Figure S7b). However, the presence of small

aggregates of metal or metal oxide nanoparticles that would

remain invisible in the X-ray pattern cannot be completely

excluded. On the contrary, the presence of Ga₂O₃aggregates in

the case of XS-GaNit-A (Figure S7c) was clearly evident,

confirming the formation of extra-framework metal oxide

particles of significant size.

Figure 3. FTIR spectra of Ga-silicate materials.

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Table 1. Physicochemical properties of Ga-silicate materials.

Material

Si/Ga^[a]

Si/Ga^[b]

Surface area

(m²g^-1)

Average pore

size (nm)

Pore volume

(cm³g^-1)

Internal pore

volume (cm³g^-1)^[c]

Number of acid

sites (mmol g^-1)^[d]

XS

-

1337

1081

1020

997

2.6

2.25

1.78

1.85

1.59

1.65

2.15

2.05

-

XS-GaLac-A

XS-GaLac-B

XS-GaCit-A

XS-GaCit-B

XS-GaNit-A

XS-Ga₂O₃-A

118

130

80

88

0.68

0.70

0.65

0.61

0.94

0.81

0.39

0.26

0.21

0.13

0.04

0.009

81

144

151

86

892

1287

1045

81

78

[a] Data determined by ICP-OES.

[b] Data determined by XPS.

[c] BJH data measured in the p/p₀range 0.0-0.8.

[d] Number of sites determined by NH₃-TPD.

Figure 4. ⁷¹Ga solid state MAS NMR spectra of (a) XS-GaLac-A,

(b) XS-GaLac-B, (c) XS-GaCit-A, (d) XS-GaCit-B, (e) XS-GaNit-A

and (f) XS-Ga2O3-A.

Figure 5. XPS spectra of Si 2p of (a) XS-GaLac-A, (b) XS-GaCit-A,

(c) XS-GaLac-B and (d) XS-GaCit-B.

All the XS-Ga-based materials present a series of promising

features for catalytic applications. However, an extremely

important parameter to be considered is the acidity of the

selected catalysts which in turn influences their catalytic

performances. Hence, the acid properties of all the synthesized

Ga-based solids were evaluated via temperature programmed

desorption (TPD) of ammonia (Table 1). The results of

ammonia-TPD reveal a clear trend in which the highest

population of acid sites is given by the GaLac-based samples.

These data support the previous hypothesis (based on the

analysis of the N₂physisorption) of a better dispersion of Ga-

based active species achieved in the presence of gallium lactate

complexes. Moreover, the thermal treatment under argon and air

was able to provide solids displaying an increased amount of

acid sites (compare samples prepared via method A and B)

most probably due to a better dispersion (or lower aggregation)

of the gallium precursors. In order to get more insight on the

dispersion of the Ga species at the surface, XPS analysis

(Figures 5, 6 and Figures S9, S10) was performed as well and

results were compared with the ICP-OES data.

Figure 6. XPS spectra of Ga LLM of (a) XS-GaLac-A, (b) XS-GaCit-A,

(c) XS-GaLac-B and (d) XS-GaCit-B.

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On the basis of the characterizations results discussed

above, it can be evidenced that mesoporous gallosilicate

materials present favorable features in the perspective of

catalytic applications. The combination of acid sites, high

surface area and the accessible pores are expected to have a

positive impact on the performance of the materials. All the

synthesized mesoporous gallosilicate materials have been

evaluated in the conversion of glycerol to solketal under solvent-

free conditions. The catalytic activities were compared in terms

of TON (turnover number) here defined as mol of solketal

produced/mol of Ga atoms. In order to have a meaningful

comparison, the TON/TOF values should be compared at the

beginning of the reaction when the reactants concentrations are

far to be depleted. For this reason, a kinetic study was

performed using XS-GaLac-A and XS-GaCit-A as catalysts. As

shown in Figure 7, in the two tested catalysts, the conversion of

glycerol tends to reach a plateau after 6 h with 10 mg of catalyst,

and after 2 h of reaction with 25 mg of catalyst. This kinetic

study shows that, at any reaction time, the catalyst XS-GaLac-A

is more active than XS-GaCit-A. Therefore, longer reaction times

were not necessary since glycerol conversion kept constant.

The kinetic study was then used to select an adequate

reaction time to compare the catalytic activity of the solids. A

reaction time of 3 h (while using 10 mg of catalyst) was selected

as the best compromise to be far enough from equilibrium

conversion. All the materials were tested under the selected

reaction conditions and the results are reported in Table 2. In

principle from the reaction between glycerol and acetone, two

products can be formed, the five membered-ring (solketal) and

the six membered-ring (acetal), depending on the acetalization

position of the glycerol molecule (scheme 1). The

catalytic

experiments carried out without catalysts under the same

reaction conditions (blank experiments using XS) showed no

glycerol conversion to solketal. From the analysis of the data

reported in Table 2 emerged that the catalysts calcined via the

method A display higher TON than the analogous solids

calcined employing method B (compare entries 2-3, 4-5, 6-7 and

8-9 respectively). This suggests that calcination under argon/air

allows the preparation of solids which are more efficient towards

the production of solketal. The difference in their catalytic

performance can be explained in terms of a better dispersion of

Ga based species (when the thermal treatment is performed

under argon/air) thus of a higher amount of well dispersed acid

sites. This observation is in agreement with the previous

investigation via NH₃-TPD. The general trend observed for all

the catalysts is in perfect agreement with the experimental data

collected through acidity measurements with

a selectivity

towards solketal >95% being successfully achieved regardless

of the calcination method used. No significant amount of six-

membered-ring was observed for all solids. The best catalytic

results (irrespective from reaction temperature selected) were

obtained with XS-GaLac-A which showed high selectivity and

high conversion of glycerol towards solketal. Blank experiments

performed in presence of Ga₂O₃nanoparticles as well as of

Ga₂O₃nanoparticles supported on mesoporous silica confirmed

the importance of having well dispersed gallium species

behaving as highly accessible single sites. Moreover, an

analogous catalytic test conducted in the presence of XS-GaNit-

A resulted in a very low solketal yield, thus confirming the

importance of the choice of the gallium precursors employed for

the impregnation.

In order to obtain additional insight on the differences

between the catalytic performances of the solids, the

hydrophilicity of the catalyst surface was considered as well.

Considering that the acetalization of glycerol implies the

formation of water as side product, more hydrophobic surfaces

are expected to favor the local removal of water molecules from

the active site, thus increasing the solketal yield. The

hydrophilicity of the most active catalysts of each series (XS-

GaLac-A and XS-GaCit-A) presented in this work was estimated

by measuring the amount of water adsorbed on the surface of

the materials using thermogravimetric analysis (TGA). The

number of adsorbed water molecules on the surface of each

catalyst (nH₂O/nm²) was calculated according to formula

mentioned in the experimental section. According to these data,

the catalyst XS-GaLac-A (nH₂O/nm²= 2.8) appears to be more

hydrophobic than XS-GaCit-A (nH₂O/nm²= 3.1). Therefore, the

Figure 7. Kinetic study performed using XS-GaLac-A and XS-GaCit-A

catalysts. a) Reaction conditions: catalyst amount 10 mg,

acetone/glycerol ratio 4/1, 50°C, b) Reaction conditions: catalyst

amount 25 mg, acetone/glycerol ratio 4/1, 50°C.

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higher hydrophilicity of XS-GaCit-A catalyst is assumed to be

responsible for the lower catalytic activity.

Table 2. Catalytic performance of Ga-silicates in the conversion of glycerol to solketal.

Entry

Catalyst

Temperature

X_glycerol

(%)

0

34

23

27

20

Y_Sk(%)

S_SK

TON^[a]

TOF^[b]

(°C)

80

50

1

2

3

4

5

XS

0

XS-GaLac-A

XS-GaLac-B

XS-GaLac-A

XS-GaLac-B

32

21

25

18

>95

>97

>95

2305

1513

1801

1297

768

504

600

432

6

7

XS-GaCit-A

XS-GaCit-B

80

27

19

25

17

>95

>93

1997

1358

665

452

8

9

10

11

12

XS-GaCit-A

XS-GaCit-B

Ga₂O₃

XS- Ga₂O₃-A

XS-GaNit-A

50

22

17

0

8

20

15

0

7

>96

>94

0

>93

1598

1198

0

570

532

399

0

190

Reaction conditions: 10 mg catalyst, acetone/ glycerol molar ratio 4/1, reaction time 3 h.

[a]TON (Turnover number) is defined as the number of moles of glycerol converted per mole of Ga atoms.

[b]TOF (Turnover frequency) is defined as the number of moles of glycerol converted per mole of Ga atoms and per hour. In both catalysts, TOF values were

calculated using the data after 3 h of reaction when the reaction is far from equilibrium conditions.

Many previous literature data report the use of various

catalysts for the acetalization reaction in presence of solvents.

For this reason, in order to allow a meaningful comparison of our

best catalysts with reference solids reported in the literature,

additional catalytic tests using different amounts of t-butanol as

solvent were performed. The results summarized in Table 3

evidence the better catalytic performance of XS-GaLac-A and

XS-GaCit-A not only with respect to previously reported Ga-

based solids (compare entries 1 and 2 with 8 and 9), but also to

some metal-containing silicates reported in literature as Sn-

MCM-41 and Al-TUD-1 (compare entries 1-2 with entries 5 and

7) or Sn-TUD-1 (compare entries 3-4 with 6). Moreover, they

display comparable activity to other highly performing solids as

Zr- and Hf-based silicates.^[19]This comparison constitutes a

proof of the outstanding performance of the acid mesoporous

materials developed in the present work which is due to a

favorable combination of catalytic features such as high surface

area, highly accessible acid sites and insertion of well- dispersed

The influence of some experimental parameters such as

catalyst amount, temperature and acetone/glycerol molar ratio

was also studied. The catalyst amount was investigated at

different temperatures (25°C, 50°C and 80°C) and some

representative tests are reported in Figure 8. The catalysts

loading were varied from 5 mg to 25 mg while the other reaction

parameters were kept constant. In the absence of catalyst, no

solketal formation was observed. As expected, decreased

solketal yields were observed at 25°C. This decrease in the

catalytic performance might also be ascribed to the intrinsic

difficulties related to the selected reaction conditions,

considering the high viscosity of glycerol which can be also

responsible for diffusional limitations. However, despite the

challenging conditions, good catalytic performances were

achieved with 25 mg of catalyst. A significant increase was

obtained at 50°C and 80°C. Under optimized conditions

employing a reaction time of 6 h and an acetone/glycerol molar

ratio 4/1, a further increase in the amount of catalyst (at a

gallium species simulating as single site in the silica architecture. selected temperature) did not affect the solketal yield. For both

temperatures (50°C and 80°C) a plateau was reached with 10

Table 3. Catalytic activity of Ga-silicates compared to metal-containing

silicates reported in literature in the conversion of glycerol to solketal.

mg of catalyst. Similar results were previously observed by

Bivona et al.^[27]as well as by Nanda et al.^[28-29]and suggest that

Y_Sk

(%)

29

S_Sk

(%)

99

Entry

Catalyst

TOF

Reference

an equilibrium conversion is reached and that further

improvements of the catalytic activity are not possible in the

absence of modifications of some parameters (e.g. reactants

addition or product removal) which can shift the equilibrium itself.

1

2

3

4

5

6

7

8

9

XS-GaLac-A^[a]

XS-GaCit-A^[a]

XS-GaLac-A^[b]

XS-GaCit-A^[b]

Sn-MCM-41^[a]

Sn-TUD-1^[b]

Al-TUD-1^[a]

140

123

159

134

102

94

This work

23

33

25

42

44

28

25

28

99

This work

99

This work

Li et al.^[19]

Li et al.^[25]

Li et al.^[19]

Collard et al.^[26]

99

61

XS-Ga-MCM-41-L^[a]

XS-Ga-MCM-41-H^[a]

>99

60

21

Reaction conditions: 0.01 mol glycerol, 0.01 mol acetone, 25 mg catalyst,

80°C for 6h and t-butanol as solvent.

[a]1. 48 g t-butanol

[b] 0.74 g t-butanol

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treatment at 500°C for 2 h. As can be clearly seen in Figure 9a

for XS-GaLac-A and in Figure S12 for XS-GaCit-A, this short

calcination step allowed recovering completely the catalytic

activity of the solids. Aiming to an improved efficiency, the

activation of the catalyst at 500°C was also performed with XS-

GaCit-A: after each run, the catalyst was centrifuged and

washed twice with ethanol and thermally treated at 500°C for 2 h

(Figure 9b). This approach was successful and allowed reusing

the catalyst in four consecutive runs with no significant activity

loss, indicating that a thermal treatment at 500°C is necessary to

remove the organic residues and fully reactivate the catalyst.

TGA experiments carried out on fresh and reused catalysts

(Figure S13) illustrate the need of a thermal regeneration after

reuse, as already reported by different authors.^[26,30]This thermal

treatment is usually required for the removal of carbonaceous

species. However, a possible influence of the thermal treatment

in the reactivation and/or regeneration of the tetrahedral

coordination of the active sites cannot be totally excluded. The

importance of the thermal treatment on the insertion of the metal

inserted as single sites was previously demonstrated in the case

of Sn-based catalysts.^[1,27]

Figure 8. Effect of catalysts amount on the yield of solketal using

a) XS-GaLac-A catalyst and b) XS-GaCit-A catalyst. (Reaction

conditions: acetone/glycerol ratio 4/1, reaction time 6 h).

An approach that can be employed to reach a higher

glycerol conversion in the condensation reaction with acetone

consists of using an excess of acetone relative to glycerol. In

this study, apart from the acetone/glycerol ratio of 2/1, three

excess ratios of 4/1, 6/1 and 9/1 were investigated. As shown in

the (Figure S11), higher solketal yields can be obtained

employing a 4/1 and 6/1 molar ratio. However, in presence of a

9/1 molar ratio, the solketal yield decreased slightly probably

because of an effect of dilution.

In order to verify the catalysts stability, the materials were

tested in several consecutive cycles employing two different

recycling approaches. The first recycling protocol only involved

washing of the catalyst (after removal from the reaction mixture)

with ethanol and drying in an oven at 150°C. However, the

solketal yield drastically dropped in the second run and

continued decreasing in the successive cycles as shown in the

four first catalytic runs with XS-GaLac-A (Figure 9a) and XS-

GaCit-A (Figure S12). Such a recycling behavior suggests that

certain amounts of residues are deposited on the catalyst during

the reaction and that complete restoration of the activity cannot

be achieved only by washing. In order to confirm this hypothesis,

after the 4^thcycle, the catalyst was submitted to a thermal

Figure 9. Recycling experiments using a) XS-GaLac-A

catalyst and b) XS-GaCit-A catalyst with reaction

conditions: 0.04 mol glycerol, 0.16 mol acetone, 40 mg

catalyst, 4 h, 50°C. In a) the catalyst was heated at 500°C

for 2 h after the 4^thcycle.

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In addition, the heterogeneous nature of the mesoporous

characterization allows correlating the structural and textural

catalysts was confirmed via

a

hot filtration test. In this

properties

of

the

solids

with

the

catalyst

experiment, the solids were removed from the reaction mixture

after 1 h. The filtrate was allowed to react for further 5 h at 50°C.

The solketal yield was analyzed by ¹H-NMR both after 1 h and at

the end of the filtration test (6 h). It was noticed that the solketal

yield remained unchanged after the additional 5 h which

demonstrates the absence of leaching of active species during

the reaction (Figure 10) and ensures (together with the recycle

experiments) the truly heterogeneous nature of the impregnated

catalysts. The absence of gallium leaching was again supported

by ICP-OES analysis on the regenerated catalysts. After the last

reuse of both catalysts, no difference was observed in terms of

Si/Ga between the fresh and the recovered catalysts (Table S2).

Moreover, the characterization of the reused catalyst via TEM

(Figure S14) did not display any substantial difference with the

fresh catalyst, confirming the stability of the mesoporous

gallosilicate materials under the selected reaction conditions.

performances. Recycling tests demonstrated that the catalysts

can be efficiently reused in multiple runs without substantial

decrease in the catalytic performance when regenerated by

thermal treatment. The stability of the gallosilicate materials was

supported via characterization of the recovered catalysts after

the last reuse.

Experimental Section

Materials

Gallium(III) nitrate hydrate (Ga(NO₃)₃·xH₂O, purity 99.9%), gallium(III)

oxide (Ga₂O₃, purity ≥ 99.99 %), lactic acid solution (purity ≥ 85 %),

ammonia solution (28.0 wt.%), acetone (purity ≥ 99.7 %) and glycerol

(purity ≥ 99 %) were purchased from Sigma Aldrich. Anhydrous citric acid

(purity ≥ 99%) was purchased from Alfa Aesar. Cetyltrimethylammonium

bromide (CTAB, purity 98.0 %) and tetraethyl orthosilicate (TEOS, purity

97.0 %) were purchased from Tokyo Chemical Industry (TCI). Absolute

ethanol (EtOH, purity ≥ 99.5 %) was purchased from Fisher Scientific.

Gallium precursors and catalysts preparation

Gallium lactate monohydrate was synthesized following a procedure

reported previously^[31]but using gallium nitrate as reactant instead of Ga

metal. Gallium hydroxide was precipitated by the addition of adequate

amount of Ga(NO₃)₃·xH₂O followed by a slight excess of aqueous

ammonia. The mixture was gently heated to expel excess ammonia and

the semi-gelatinous precipitate was collected by centrifugation and

washed twice with cold water. Freshly precipitated gallium hydroxide was

mixed with concentrated lactic acid. The mixture was diluted with water,

stirred and heated on a steam-bath. The excess gallium hydroxide was

filtered off and the clear filtrate evaporated on a steam-bath to a viscous

liquid at point which white plates of gallium lactate started to appear.

Anhydrous acetone was added to the syrupy mixture to dissolve the

excess lactic acid and to precipitate the dissolved salt. The gallium

lactate was washed once with acetone. The product after vacuum

desiccation was an amorphous white powder. Yield (94.5%). Elemental

analysis calcd (%) for C₉H₁₇O₁₀Ga: C 32.01, H 4.8, Ga 20.7, found: C

33.0, H 4.5, Ga 21.3.

Figure 10. Hot filtration tests using XS-GaLac-A and XS-GaCit-A

catalysts. (Reaction conditions: 0.01 mol glycerol dissolved in 0.7 mL

of absolute EtOH, catalyst amount 25 mg, 0.04 mol acetone, 50°C).

Conclusions

A series of mesoporous gallosilicate materials was successfully

synthesized via wet impregnation procedure employing two non-

conventional Ga-complexes of different polarities. All XS-Ga

materials consist of nanosized silica particles and display a

remarkably high surface area and a narrow pore size distribution.

The conversion of each gallium atom in a catalytically active site

was proved via a combination of characterization tools including

solid state ⁷¹Ga NMR. All the solids were tested as

heterogeneous catalysts in the conversion of glycerol to solketal.

The most active catalyst (XS-GaLac-A) displayed outstanding

turnover numbers compared to other Ga-based silicates

prepared via co-synthesis approach. The excellent catalytic

activity can be ascribed to the synthesis strategy which

generates fully accessible and highly dispersed surface gallium

Ammonium dicitratogallate(III) dihydrate was synthesized following a

procedure reported previously.^[32]A mixture of Ga(NO₃)₃.xH₂O and

anhydrous citric acid was dissolved in water. The resulting slurry was

stirred at 50°C overnight. The color of the solution remained clear

throughout that period. The water was removed by means of a rotary

evaporator. The produced gummy material was redissolved in

a

minimum amount of water, and the pH was adjusted to 8 with aqueous

ammonia. The addition of absolute ethanol at 4°C yielded crystalline

material which was isolated by filtration. Yield (83.9%). Elemental

analysis calcd (%) for C₁₂H₃₂O₁₆N₅Ga: C 25.2, H 5.6, N 12.2, Ga 12.2,

found: C 25.9, H 5.6, N 13.2, Ga 12.6.

The general protocol for the synthesis of the XS-MCM-41 is inspired by a

previously reported method.^[22]CTAB surfactant (1.38 g) was dissolved in

a propylene bottle containing milli-Q water (628.98 g) under 800 rpm

species simulating

a single-site. Moreover, the in-depth

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stirring at room temperature.

A

concentrated aqueous solution of

The silica-supported catalysts were characterized by powder X-ray

diffraction, surface area determination, transmission electron microscopy,

ICP-OES elemental analysis, X-ray photoelectron spectroscopy, Fourier

transform infrared spectroscopy, ⁷¹Ga solid state NMR, ammonia

temperature-programmed desorption and TGA analyses.

ammonia (2 g) was added slowly to the mixture. After 30 min, TEOS (6 g)

was added dropwise to the basic micellar solution under the same

conditions. The white solid was filtered and washed three times by milli-Q

water and ethanol. The product was then dried overnight in an oven at

65°C. Finally, the resulting solid was calcined in air to remove the organic

template (5 h at 550°C, heating rate of 3°C min^-1).

Powder X-ray diffraction (XRD) patterns were measured on a PANalytical

X’pert pro diffractometer with Cu Kα radiation (λ = 1.54178 Ǻ).

Gallosilicate materials were prepared via wet impregnation. XS-GaLac-

MCM-41 and XS-GaCit-MCM-41 were prepared by dissolving 14 mg of

GaLac and 24 mg of GaCit respectively, in 350 µL of water with 2 mL of

ethanol in a 10 mL round bottom flask. Then, 200 mg of XS-MCM-41 was

added to the solution and stirred for 90 min at room temperature. The

solvent was removed under reduced pressure.The solids were calcined

under two different conditions: (i) under argon at 550°C (heating rate of

2°C min^-1) then in air for 5 h and (ii) under air only at 550°C for 5 h, 2°C

min^-1. The materials calcined under argon/air were denoted as XS-

GaLac-A and XS-GaCit-A, while those calcined only under air were

denoted as XS-GaLac-B and XS-GaCit-B. For comparison, other batches

of XS-GaNitrate-MCM-41 and XS-Ga₂O₃-MCM-41 were synthesized. The

materials were only calcined under argon/air and denoted as XS-GaNit-A

and XS-Ga₂O₃-A.

Specific surface area and porosity of the solids were determined from the

nitrogen adsorption–desorption isotherms obtained at 77

K with a

volumetric adsorption analyzer (Micromeritics Tristar 3000). The samples

were pre-treated at 150°C for 24 h under reduced pressure (0.1 mbar).

The Brunauer–Emmett–Teller (BET) method was used to calculate the

specific surface areas in the p/p₀= 0.05-0.30 range. The Barrett–Joyner–

Halenda (BJH) method applied to the adsorption isotherm was used to

determine the pore size distributions.

Transmission electron microscopy (TEM) images were recorded on a

Philips Tecnai 10 with an accelerating voltage of 100 kV. Samples were

prepared by dispersion of a small quantity of material in absolute ethanol

and deposited onto a copper grid.

The quantity of gallium was quantified by ICP-OES analysis. The

analysis was performed by dissolving 10 mg of each sample in a mixture

of acids (HF+HCl+HNO₃). The obtained solutions were analyzed using

an Optima 8000 ICP-OES Spectrometer.

Characterization of the materials

Gallium(III) precursors were characterized on the basis of elemental

analysis, infrared, TGA, ¹³C liquid NMR spectroscopy and ⁷¹Ga liquid

NMR spectroscopy.

The ⁷¹Ga solid state NMR MAS spectra were recorded at room

temperature on a Bruker Avance-500 spectrometer operating at 11.7 T

using a 2.5 mm Bruker CP-MAS probe. The solids were prepared in a

2.5 mm zirconia rotor and analyzed with a spinning frequency of 25000

Hz. The spectra were recorded with a Hahn echo sequence and the

following acquisition parameters: a relaxation time of 0.3 s, an excitation

of 2.6 μs (90°) and an acquisition time of 5 ms. Data processing includes

the multiplication of the FID (Free Induction Decay) by a line broadening

factor of 700 Hz, zero-filling, Fourier transformation, phase and baseline

corrections. The chemical shift scale was calibrated at room temperature

using Ga(NO₃)₃as the reference compound (0 ppm).

C–H–N elemental analyses were carried out at the University College of

London.

IR spectra in the 4000–400 cm⁻¹range were recorded on a FTS-135 Bio-

RAD spectrometer, using KBr pellets containing ca. 1 wt.% of the powder.

TGA analyses were performed in air at the heating rate of 10°C min^-1

using a Mettler Toledo TGA/SDTA851^eanalyzer.

The ¹³C liquid NMR spectra were recorded at 100.63 MHz in D₂O by a

JEOL 400 MHz spectrometer and the chemical shift scale was calibrated

The XPS analyses were carried out with a SSX 100/206 photoelectron

spectrometer from Surface Science Instruments (USA) equipped with a

monochromatized micro focused Al X-ray source (powered at 20 mA and

10 kV). The powdery samples were fixed with double-sided tape onto

small brass troughs of 6 mm diameter that were placed on a ceramic

carousel. The pressure in the analysis chamber was around 10^-6Pa. The

angle between the surface normal and the axis of the analyser lens was

55°. The analyzed area was approximately 1.4 mm²and the pass energy

was set at 150 eV. In these conditions, the full width measured at half

maximum (FWHM) of the Au 4f_7/2peak for a clean gold standard sample

was about 1.6 eV. A flood gun set at 8 eV and a Ni grid placed 3 mm

above the sample surface were used for charge stabilization. The

following sequence of spectra was recorded: survey spectrum, C 1s, O

1s, Si 2p, Ga LLM and C 1s again to check the stability of charge

compensation with time. The C-(C,H) component of the C1s peak of

carbon has been fixed to 284.8 eV to set the binding energy scale. Data

treatment was performed with the CasaXPS program (Casa Software Ltd,

at RT with respect to

a

sample of solid 3-(trimethyl-silyl)-1-

propanesulfonic acid sodium salts (DSS; δ= 0.0 ppm).

The ⁷¹Ga NMR spectra were recorded using RIDE (ring down

elimination) pulse sequence on the Varian VNMRS400 with the following

acquisition parameters: spectral width of about 940 ppm (83.3 kHz), 15

ms relaxation delay, 5 ms acquisition time and a number of transients

ranging between 4.0 × 10³and 8.2 × 10⁴. The spectra were recorded

lock-on without sample spinning. The processing comprised exponential

multiplication of the FID with a line broadening factor of 100 Hz, zero-

filling, Fourier transform, zero-order phase correction, baseline correction,

and correction of the free induction decay (FID) by backward linear

prediction. The ⁷¹Ga chemical shift scale was calibrated at 298 K with

respect to an external sample of Ga(NO₃)₃in D₂O (0 ppm) and the pH

was adjusted by addition of NaOH or HCl solution.

10.1002/cctc.202001172

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UK), some spectra were decomposed with the least squares fitting

routine provided by the software with a Gaussian/Lorentzian (85/15)

product function and after subtraction of a non-linear baseline. Molar

fractions were calculated using peak areas normalized based on

acquisition parameters and sensitivity factors provided by the

manufacturer.

centrifugation. Afterwards, the solid was dried overnight at 150°C and

reused in a new catalytic run. After the forth reuse, the catalyst was

calcined at 500°C for 2 h (heating rate of 2^°C min^-1). In case of XS-GaCit-

A, the catalyst was directly calcined at 500°C for 2 h (heating rate of 2°C

min^-1).The subsequent catalytic tests were carried out by repeating this

procedure from the beginning (the quantities were adapted in function of

the mass of the recovered catalyst).

Temperature-programmed desorption of ammonia (NH₃-TPD) was

carried out to evaluate the total acidity of the catalysts. Around 50 mg of

sample was introduced in a quartz reactor on a Hiden Analytical Catlab-

PCS apparatus. Prior to measurement, the sample was preheated in

argon flow (30 mL min^-1) at 200°C for 2 hours. Subsequently, it was

cooled down to 50°C and then exposed to a gas mixture of 5% NH₃in He

(10 mL min^-1) and Ar (20 mL min^-1) for 45 min. Physisorbed NH₃was

removed by purging with Ar (30 mL min^-1) at 50°C for 90 min. The TPD

measurement was conducted by heating the sample from 50 to 650°C

with ramping rate of 5°C min^-1. Desorbed NH₃was detected by a mass

spectrometer (QGA model).

Leaching tests were performed at 50°C by dissolving glycerol (0.01 mol)

in 0.7 mL of absolute ethanol with 2 min sonication followed by addition

of 25 mg of catalyst and acetone (0.04 mol). The catalyst was removed

from the reaction mixture after 1 h by hot filtration using a plastic syringe

equipped with a 25 mm syringe filter, with a pore size of 0.2 µm, then

followed by centrifugation at the same temperature. The filtrate was

allowed to react for another 5 h. The reaction mixture was analyzed by

¹H-NMR after 1 h and at the end of the filtrate test (6 h).

Thermogravimetric analyses (TGA) were carried out under inert

atmosphere (nitrogen gas flow of 90 mL min⁻¹) using a Mettler Toledo

TGA/SDTA851^eanalyzer, with a heating rate of 10°C min⁻¹in the range

25-900°C. Following a previously reported procedure^[33]all samples were

pre-treated overnight in a desiccator containing a saturated aqueous

solution of NH₄Cl for 72 h to reach the maximum level of water

adsorption. The hydrophilicity of the materials was estimated from the

number of water molecules adsorbed on the surface of each sample,

calculated from the mass loss between 25 and 200°C measured by TGA,

using the following equation:

Acknowledgements

The authors acknowledge the “Communauté française de

Belgique” for financial support, including the PhD fellowship of H.

Hussein, through the ARC programme PolarCat (15/20-069).

This research used resources of the nuclear magnetic

resonance service of the “Plateforme Technologique Physico-

Chemical Characterization” – PC², located at the University of

Namur.

∆푚

푁_A

푛_H

=

×

O

2

푚_iS_BET× 푀_H

O

2

Keywords: mesoporous gallosilicates • glycerol • solketal

where nH₂O is the number of adsorbed water molecules per nm²of

catalyst surface; Δm is the mass loss between 25 and 200°C (g); m_iis

the initial mass of the sample at 25°C (g); MH₂O is the molar mass of

water (18.0153 g mol⁻¹); S_BETis the surface area calculated from the

nitrogen adsorption-desorption isotherms (nm²g⁻¹); N_Ais the Avogadro

constant.

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10.1002/cctc.202001172

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Mesoporous gallosilicate

catalysts synthesized via

wet impregnation exhibit

Hussein Hussein,^[a]Alvise

Vivian,^[b]Luca Fusaro,^[b]Michel

Devillers,*^[a]Carmela Aprile*^[b]

outstanding

catalytic

activity in the direct

conversion of glycerol to

solketal under solventless

conditions.

Page No. – Page No.

Synthesis of highly

accessible gallosilicates via

impregnation procedure:

enhanced catalytic

performances in the

conversion of glycerol into

solketal

Article Doi

Synthesis of Highly Accessible Gallosilicates via Impregnation Procedure: Enhanced Catalytic Performances in the Conversion of Glycerol into Solketal

DOI: 10.1002/cctc.202001172

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Article abstract of DOI:10.1002/cctc.202001172

Full text of DOI:10.1002/cctc.202001172

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