Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Unprecedented oxidative properties of mesoporous silica materials:
Towards microwave-assisted oxidation of lignin model compounds
Sushanta K. Badamali a,b,⁎, Rafael Luque b,c, James H. Clark , Simon W. Breeden
b
b
a
Department of Chemistry, Utkal University, Vani Vihar, Bhubaneswar, Odisha 751 004, India
b
Green Chemistry Centre, Department of Chemistry, University of York, Heslington, York, YO10 5DD, United Kingdom
Departamento de Quimica Organica, Edificio Marie Curie (C-3), Campus de Rabanales, Ctra Nnal IV-A, Km 396, Cordoba, Spain
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The unusual oxidative ability of mesoporous silicas towards oxidation of an important lignin model molecule,
,2-(4-hydroxy-3methoxy-phenoxy) ethanol, apocynol under microwave irradiation is presented in this
work. Mesoporous MCM-41, HMS, SBA-15 and amorphous silica were employed as catalysts in the present
study. Different reactivities were obtained for the various silica materials. It was assumed that the substrate
conversion and product selectivity were highly influenced by the nature of mesoporous silica materials.
Based on the nature of the catalysts and reaction product profile, a plausible mechanism has been proposed.
Received 6 September 2012
Received in revised form 31 October 2012
Accepted 2 November 2012
Available online 9 November 2012
1
Keywords:
Mesoporous silica
Microwaves
©
2012 Elsevier B.V. All rights reserved.
Apocynol
Lignin degradation
1
. Introduction
employs conventional silicates as catalysts without any of the typical
active species reported in redox heterogeneous catalysis [7,8]. We
have utilized three representative ordered mesoporous silicas such as
MCM-41, HMS and SBA-15 as catalysts. For comparative purposes, we
have used commercial grade mesoporous silica as catalyst, which
is amorphous in nature. The molecule studied [1,2-(4-hydroxy-
3methoxy-phenoxy) ethanol] is an important lignin model phenolic
monomer which has been largely investigated in order to understand
the reactivity of lignin and lignin derived molecules. Recently, we
have demonstrated that functionalized mesoporous silica materials
can be used as efficient catalysts in the selective oxidation of the mono-
mer [9,10] under microwave activation.
Mesoporous silicate materials have gained an increased attention
in the past decades owing to their interesting properties that include
tunable pore sizes, stabilities and shape selectivity [1]. These mate-
rials have extensively been employed in a range of applications in-
cluding adsorbency, catalysts, support and advanced materials [2].
Commonly, the basic silicate structures have been modified by
inserting catalytically-functional groups such as metal ions or metal
complexes in order to improve their adsorption and catalytic proper-
ties. The large abundance of internal surface hydroxyl groups facili-
tates anchoring/grafting catalytically active groups. Pure silica or
silicates displaying catalytic behavior are rather unusual due to the
low acidity of silanol groups and have been reported only in excep-
tional cases [3]. Morasas and Harrington have reported the oxidation
behavior of quartz towards aerial oxidation of organic compounds [4]
but the reaction mechanism is still unclear [5]. In a recent report, Itoh
et al. have also studied the photocatalytic decarboxylation of organic
compounds catalyzed in the presence of mesoporous silicas [6].
In this work, we present the unusual oxidation ability of mesoporous
silica materials towards a lignin model compound (apocynol) under mi-
crowave irradiation. The proposed approach is highly promising as it
2. Experimental
Mesoporous silica catalysts; MCM-41, HMS and SBA-15 were pre-
pared according to previously reported protocols [11–13]. Commer-
cial grade mesoporous silica (Silica-5, Grace, Germany) was also
used as catalyst. Apocynol was synthesized and characterized follow-
ing the procedure outlined elsewhere [6]. Acetovanillone and vanillin
(Aldrich) and 2-methoxybenzoquinone (TCI) were used as received.
Materials were characterized using X-Ray powder diffraction (XRD),
TG-DTA and nitrogen physisorption measurements as described else-
where [11–13]. ICP/MS experiments were conducted with typically
2
3
5 mg of sample digested in a mixture of aqueous HF, HNO and
⁎
Corresponding author at: Department of Chemistry, Utkal University, Vani Vihar,
HCl at 1:1:1 ratio. Upon dissolution, the sample was transferred to a
250 mL calibrated flask, diluted with miliQ water (to a maximum