DOI: 10.1002/cssc.201500124
Communications
Hierarchical Zeolites and their Catalytic Performance in
Selective Oxidative Processes
[
a]
[b]
[a]
[a]
Manuel Ojeda, Aida Grau-Atienza, Rafael Campos, Antonio A. Romero,
[b]
[a]
[b]
[a]
Elena Serrano, Jose Maria Marinas, Javier Garcꢀa Martꢀnez,* and Rafael Luque*
[
23–25]
Hierarchical ZSM-5 zeolites prepared using a simple alkali treat-
ment and subsequent HCl washing are found to exhibit unpre-
cedented catalytic activities in selective oxidation of benzyl al-
cohol under microwave irradiation. The metal-free zeolites pro-
mote the microwave-assisted oxidation of benzyl alcohol with
hydrogen peroxide in yields ranging from 45–35% after 5 min
of reaction under mild reaction conditions as well as the epoxi-
dation of cyclohexene to valuable products (40–60% conver-
sion). The hierarchically porous systems also exhibited an inter-
esting catalytic activity in the dehydration of N,N-dimethylfor-
mamide (25–30% conversion), representing the first example
of transition-metal free catalysts in this reaction.
ly useful for the petrochemical industry.
In this regard,
both microporous and hierarchical (mesoporous) zeolites have
been extensively employed in a range of acid-catalyzed pro-
cesses, including alkylations and acylations as well as, more re-
[23,26–28]
cently, transformations of biomass-derived compounds.
Other microporous zeolites (e.g., ZSM-5) have also been re-
ported in selected transition-metal-free alkane functionaliza-
[29]
tions. Interestingly, zeolites have rarely been reported in se-
lective oxidation processes. Pescarmona and Jacobs disclosed
the first and only remarkable example of good catalytic activity
for zeolites beta and Y, with Si/Al ranging from 2.6 to 15, in
alkene epoxidation. Under optimized conditions, a maximum
cyclooctene conversion of 60% with an 82% conversion to the
epoxide was achieved under mild reaction conditions (808C,
4 h). The authors related this unusual epoxidation activity in
USY to the formation of surface hydroperoxide species (Alꢀ
OOH) from the reaction of AlꢀOH groups with hydrogen per-
[30]
Zeolites form an important family of microporous crystalline
aluminosilicates and have been the subject of extensive stud-
ies. Owing to properties such as high crystallinity, hydrother-
mal stability, and acidity, they have found application in fields
[31]
oxide as previously reported for Al O .
2
3
[
1]
including adsorption and catalysis. However, the microporous
crystalline frameworks impose a series of inherent limitations,
The development of mesoporosity in hierarchical zeolites
can open up the porous framework in these materials, and
induce the formation of a large number of accessible terminal
[
2–5]
mostly related to diffusional constraints.
[3]
[32]
Recent efforts have focused on the development of meso-
porosity in zeolites, which can widen the applicability of zeo-
AlꢀOH groups. Liu et al. recently reported the catalytic per-
formance of alkali-treated ZSM-5 zeolite in the selective oxida-
tion of benzylalcohol with hydrogen peroxide, pointing out
that the reaction mechanism involves the formation of Alꢀ
OOH groups by direct reaction of AlꢀOH sites with H O . We
[
6–20]
lites in reactions with bulky substrates.
Desilication by
alkali treatment is a simple and extensively investigated ap-
[
6–11,21]
proach to introduce mesoporosity.
The procedure is gen-
2
2
erally followed by an acid wash to remove the alumina debris
recently reported the synthesis and characterization of iron
oxide-functionalized hierarchical ZSM-5 zeolites with different
Si/Al ratios. The surface acid properties of these catalysts were
investigated and correlated to the catalytic activity of these
materials in the microwave-assisted alkylation of toluene with
[
22]
formed after silica extraction.
Acidity is an important property of zeolites. The presence of
aluminum leads to the formation of Brønsted (SiꢀOHꢀAl) and
Lewis (coordinately unsaturated aluminum) sites. These pro-
vide various possibilities in acid-catalyzed processes, particular-
[33]
benzyl chloride, preferentially catalyzed by Lewis acids.
In light of these findings, we set out to investigate the feasi-
bility of using these hierarchical zeolites, with and without iron
oxide nanoparticles, in selective oxidation processes taking ad-
vantage of terminal AlꢀOH groups to generate redox sites in
zeolites. Mesoporous ZSM-5 materials were consequently syn-
thesized and employed in various selective oxidation test reac-
tions under microwave irradiation, namely the oxidation of
benzyl alcohol to benzaldehyde, the epoxidation of cyclohex-
ene, and the multistep conversion of N,N-dimethyl formamide
(DMF) into products of interest.
[
a] Dr. M. Ojeda, R. Campos, Prof. A. A. Romero, Prof. J. Maria Marinas,
Prof. R. Luque
Departamento de Quimica Organica
Universidad de Cordoba
Campus de Rabanales, Edificio Marie Curie (C-3)
Ctra Nnal IV-A, Km 396, 14014 Cordoba (Spain)
Fax: (+34)957212066
E-mail: q62alsor@uco.es
[
b] A. Grau-Atienza, Dr. E. Serrano, Prof. J. Garcꢀa Martꢀnez
Laboratorio de Nanotecnologꢀa Molecular
Departamento de Quꢀmica Inorgꢁnica
Universidad de Alicante
The textural and surface acid properties of zeolite materials
synthesized in this work are summarized in Table 1. All hier-
archical ZSM-5 zeolites exhibited type I+IV isotherms typical
of micro- and mesoporous materials, as compared to the type I
isotherms observed for parent microporous zeolites (Z40c and
Ap. 99, 03080, Alicante (Spain)
Fax: (+34)965903454
E-mail: j.garcia@ua.es
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500124.
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