C. Gonzalez-Arellano et al. / Journal of Molecular Catalysis A: Chemical 406 (2015) 40–45
41
cations in the food, pharmaceutical and oleochemical industries,
for example, Guerrero-Urbaneja et al. [26] obtained polyglycerols
from glycerol etherification with oxides derived from hydrotal-
cites MgFe. Benzyl alcohol has been used as solvent for hair dyes,
but this compound may cause skin allergies [27]. A more suit-
able alternative would be the derivatives of benzyl alcohol with
glycerol, more specifically monosubstituted. Pico et al. [28] and
da Silva et al. [29] investigated the etherification of glycerol with
benzyl alcohol using different catalyst acid ion-exchange resins,
acid-functionalized mesostructured silica and zeolites, obtaining
good results in most cases.
Recently, we have reported that hierarchical ZSM-5 zeolites
prepared using a simple alkali treatment and subsequent HCl
washing exhibit unprecedented catalytic activities in selective oxi-
dation of benzyl alcohol under microwave irradiation [30]. The
scope of this article is to study the influence of Si/Al ratio and the
micro/mesoporosity of zeolites on the catalytic activity of Z15c and
Z40c zeolites on the glycerol etherification reaction with benzyl
alcohol.
saturation from where the peaks of the probe molecules in the
gas phase are detected in the GC. The quantity of probe molecule
adsorbed by the solid acid catalyst can subsequently be easily quan-
tified. In order to distinguish between Lewis and Brönsted acidity,
the assumption that all DMPY selectively titrates Brönsted sites
(methyl groups hinder coordination of nitrogen atoms with Lewis
acid sites) while PY titrates both Brönsted and Lewis acidity in the
materials was made. Thus, the difference between the amounts of
PY (total acidity) and DMPY (Brönsted acidity) adsorbed should
correspond to Lewis acidity in the materials. Diffuse Reflectance
Fourier-Transform Infrared (DRIFT) spectra of adsorbed pyridine
(PY) were carried out in an ABB IR-ATR instrument equipped with
an environmental chamber. PY was adsorbed at room temperature
for a certain period of time (typically 1 h) to ensure a complete
saturation of the acid sites in the catalyst and then spectra were
◦
recorded at different temperatures ranging from 100 to 300 C in a
similar way to previous reports [32]. With this purpose, the differ-
ent types of acid sites in the materials (Brönsted and Lewis) could
be measured and quantified.
2
. Experimental
2.3. Catalytic experiments
2
.1. Preparation of the catalysts
a) Materials: Glycerol (100% purity), supplied by Aldrich, and
benzyl alcohol (≥99.5% purity), provided by Sigma–Aldrich, were
employed as reactants. All chemicals were used without further
purification. Ethanol (≥99% purity), supplied by Scharlau, was
used as the solvent for sample analysis. Hexadecane (≥99% purity,
Sigma–Aldrich) was employed as the internal standard compound
in the chromatographic analysis. Commercial (± ) 3-benzyloxy-
1,2-propanediol (≥97% purity), 1,3-dibenzyloxy-2-propanol (≥97%
purity), and benzyl ether (≥98% purity) were used to calibrate the
gas chromatograph, and supplied by Aldrich.
Two commercial MFI zeolites in their ammonium form,
CBV8014 (Zeolyst International, Si/Al molar ratio = 40) and
CBV3024E (Zeolyst International, Si/Al molar ratio = 15), were
transformed into the acid form by calcination at 550 C for 5 h in
◦
◦
◦
−1
air (heating rate up to 550 C of 100 C h ) and used as starting
materials. These are denoted as Z40c and Z15c, in which Z stands
for the type of zeolite (ZSM-5), the number (40 or 15) refers to the
Si/Al ratio according to the manufacturer’s specifications, and the
letter c refers to the calcined material.
Hierarchical Z40c zeolite was prepared by desilication of the
zeolite Z40c using a simple alkali treatment, followed by an acid
wash with HCl [30]. Desilication treatment of Z40c zeolite was con-
ducted following the methodology reported by Verboekend et al.
b) Typical procedure for etherification reactions: In a typical
run, 10 mmol glycerol, 10 mmol benzyl alcohol and the catalyst
(0.025–0.100 g) were placed in a ampoule with continuous stirring
◦
for 8 or 15 h at 120–140 C. The resultant mixture was filtered off,
extracted using ethanol and the products were identified by GC–MS
and their ratios also. The response factors of the benzyl alcohol
was determinated using hexadecane as external standard and cal-
ibrate curve. Commercial (± ) 3-benzyloxy-1,2-propanediol (MBG)
and 1,3-dibenzyloxy-2-propanol were employed to obtained the
corresponding response factor. Dibenzyl ether was calibrated as
well. Quantification of glycerol was performed taking into account
the molar ratio on the chromatographic analysis of compounds
derived from glycerol. Any dehydration products of glycerol or of
benzyl alcohol were detected. The stirring speed was 1200 rpm to
guarantee the absence of external mass-transfer resistance.
c) Analytic methods: The composition of the reaction mixture
[
31] in which the zeolite Z40c is treated with 100 mL NaOH solution
◦
.8 M and for Z15c for 30 min at 65 C and 600 rpm. The solution
0
was then filtered off and thoroughly washed with distilled water.
Desilicated zeolite was subsequently treated in 100 mL of 0.1 M
aqueous HCl during 6 h to remove debris alumina and obtain the
ZSM-5 porous zeolites, as recently reported elsewhere [30]. Hier-
archical Z40c catalysts after acid washing is denoted as Z40c-H (see
ref. [30] for further details).
2.2. Materials characterization
(
ethers and benzyl alcohol) was analyzed by means gas chro-
The porosity of the materials was measured by N2 adsorp-
matography using an Agilent turbo system 5975 chromatograph,
integrated with a mass detector 7820A. An HP 5MS capillary chro-
matographic column (30 m × 0.25 mm × 0.25 m) was utilized. The
chromatographic conditions were as follows: initial oven tem-
perature of 50 C, final oven temperature of 230 C, and one
programmed rate of 10 C min . Retention times: peak at 6.20 min
tion/desorption isotherms at 77 K in an AUTOSORB-6 apparatus.
−
5
Samples were previously degassed for 5 h at 373 K at 5 × 10 bar.
BET surface area was estimated by using multipoint BET method
and the adsorption data in the relative pressure (P/P ) range of
0
◦
◦
0
.05–0.30. The pore size distribution was calculated from the
◦
−1
adsorption branch of the N2 isotherms using the DFT method. The
mesoporous volume was calculated from the cumulative pore vol-
ume distribution curve. Micropore volume was calculated using
the t-plot method. Pyridine (PY) and 2,6-dimethylpyridine (DMPY)
benzyl alcohol, at 13.36 min (± ) 3-benzyloxy-1,2-propanediol
(MBG), at 13.74 min hexadecane, at 14.48 min dibenzyl ether (DBz)
and at 20.43 min 1,3-dibenzyloxy-2-propanol (DBG).
◦
titration experiments were conducted at 200 C via gas phase
adsorption of the basic probe molecules utilizing a pulse chromato-
graphic titration methodology. Briefly, probe molecules (typically
3. Results and discussion
1
–2 L) were injected in very small amounts (to approach condi-
Table 1 summarizes the bulk Si/Al ratio, porosity and acidic
properties of both conventional zeolites as (Z15c and Z40c) well
as of the hierarchical Z40c-H sample, all after calcination. As pre-
viously reported [30], the introduction of mesoporosity in ZSM-5
tions of gas chromatography linearity) into a gas chromatograph
through a microreactor in which the solid acid catalyst was pre-
viously placed. Basic compounds are adsorbed until complete
Gonzalez-Arellano, Camino
