M. Selvaraj et al. / Applied Catalysis A: General 415–416 (2012) 17–21
19
Among the all Cr-containing 2D mesostructured catalysts,
CrSBA-15(8) and CrSBA-15(50) produce the highest and lowest
BP O selectivity, respectively. From this illustration, we have used
the catalysts viz. CrSBA-15(8) and CrSBA-15(50) as reusable cata-
lysts. Initially, the CrSBA-15 catalysts used for this reaction usually
suffer from their catalytic activities, and hence the catalysts need
and CrSBA-15(50) were washed four times with acetone and dried
at 120 ◦C overnight. Finally the catalysts were calcined at 500 ◦C
CrSBA-15(50) were reused for this reaction (Table 1). After the
loss of catalytic activity in first two runs (not shown in Table 1)
with the leaching of polychromate and pentavalent chromium
species from the catalytic surface (Table 1), the DPM conversion
as well as BP O selectivity remains constant after four runs, indi-
cating that the chromium species cannot be further leached on the
mesoporous matrix, and there is no polychromate and pentava-
lent chromium species on the surface of CrSBA-15 catalysts, which
are in good agreement with ICP-AES results of filtrate solutions
where no chromium ions were detected. It can be clearly observed
by the UV–vis DRS results that the leaching of chromium ion is
not observed after four runs [27]. We suspect that the aqueous t-
butanol generated as a byproduct in this reaction is responsible
for the extraction of a small amount of loosely bound pentavalent
chromium (extra-framework) in CrSBA-15 catalysts. Furthermore,
the washed CrSBA-15(8) was also used in this catalytic reaction
to find its catalytic activity and gives almost the similar results
of CrSBA-15(8) used for 4th run, as shown in Table 1. As an
important point, the non-framework chromium oxides (Table 1)
such as polychromate and Cr(VI) oxide are highly toxic and were
completely removed by washing treatments. Based on the results
obtained by reusable catalysts, CrSBA-15(8) is found to be the
best reusable and eco-friendly catalyst among the other CrSBA-15
catalysts.
Since the mesoporous CrSBA-15(8) gives the best catalytic activ-
ity for this catalytic reaction among the other 2D mesostructured
Cr-containing catalysts, it has been further extensively used with
different reaction parameters such as temperature, time and ratios
of reactants. The oxidation of DPM is carried out at different reac-
tion temperatures and times over CrSBA-15(8), as shown in Fig. 1.
When the reaction temperatures and times are increased from 313
as BP O selectivity increases, however, the increase in DPM con-
version and BP O selectivity can be linearly observed when the
reaction temperature or time is increased to 420 K and 24 h, as
shown in Fig. 1(a) and (b). These catalytic results clearly show
that the catalytic activity of CrSBA-15(8) under the same reaction
conditions has not been affected by the higher temperature and
time. When this reaction is carried out with 1:2 ratio of DPM to
TBHP, the DPM conversion (61.2%) and BP O selectivity (94.7%) can
be observed because of the equilibrium of the reactants with the
greater chemisorption on the active sites of catalyst surfaces, and
in other ratios of DPM to TBHP like 1:1, 1:3 and 2:3, the conversion
as well as selectivity decreases as shown in Fig. 2. It is clearly con-
sidered that the DPC-OH selectivity increases in the order of ratios
2:3 > 1:1 > 1:3 and observed that in the reactant ratios the acid sites
of catalyst are helpless to produce a higher BP O selectivity. A pos-
sible reason is that the quantity of reactants is insufficient to react
with each other on the catalyst surface while the active sites of
inner pore size of the catalyst may be screened by the unreacted
organics that make the sintering effects on the surface of catalysts.
Compared the results of all catalytic parameters, it can be notewor-
thy observed that the DPM oxidation at 393 K for 10 h with 1:2 ratio
of DPM to TBHP over CrSBA-15(8) gives a higher DPM conversion
as well as BP O selectivity.
O
CrSBA-15
Chlorobenzene,
393 K,
TBHP
DPM
BP=O
Scheme 1. Oxidation of DPM over CrSBA-15.
the washed CrSBA-15(8) is slightly reduced because of removal of
non-framework of chromium species on its surface pore walls [27].
3.2. Liquid phase oxidation of DPM
radical-chain mechanism [28]. In this reaction diphenylmethane
hydroperoxide (DPMHP) primarily forms on the catalytic surface
of CrSBA-15. The DPMHP decomposes in the presence of suitable
catalytic active sites on CrSBA-15 into BP O (Scheme 1), as a major
product with diphenymethanol (DPC-OH), as minor byproduct.
Due to unexpected active conditions, only very trace amounts of
unidentified products are obtained.
The oxidation of DPM to BP O has been conducted over
mesoporous chromium-containing catalysts, as shown in Table 1.
This reaction also produces DPC-OH, as minor byproduct with
very trace amount of unidentified byproduct. The BP O selectivity
increases in the following catalysts order, CrSBA-15(8) > CrSBA-
15(16) > CrSBA-15(20) > CrSBA-15(WF) > CrMCM-41(40) > CrSBA-
15(50) > CrSBA-15(0.04F) > CrSBA-15(0.07F). Among the all
Cr-containing mesoporous catalysts, CrSBA-15(8) exhibits the
best performance with a DPM conversion (61.2%) as well as BP
O
selectivity (94.7%). Very trace amount of minor byproduct (DPC-OH
selectivity, 5.3%) also forms in this reaction. CrSBA-15(8) synthe-
sized by pH-aDH method has higher selectivity of BP O than other
CrSBA-15 catalysts. The higher activity of CrSBA-1(8) observed in
this reaction is tentatively ascribed to its two-dimensional space
and high loading of tetrahedral Cr5+/Cr6+ species on the inner
pore walls of SBA-15 because the tetrahedral Cr5+/Cr6+ species
produces high numbers of accessible active on the inner surface
of pore walls. In CrSBA-15 catalysts synthesized by NH4F method,
CrSBA-15(0.04F) has higher DPM conversion than CrSBA-15(0.07F)
due to the higher loadings of chromium-content, but CrSBA-
15(WF) has much higher DPM conversion and BP O selectivity as
compared to CrSBA-15(0.04F) due to the higher chromium species
tetrahedrally coordinated into the silica pore walls, and also the
CrSBA-15(WF) has much higher DPM conversion as well as BP
O
selectivity than some Cr-containing mesoporous catalysts such as
CrSBA-15(25) and CrSBA-15(50), which have much less tetrahedral
NH4F method give more DPC-OH selectivity than other CrSBA-15
walls. When the similar chromium content of 2D mesostructured
catalysts (Table 1) have been used in this reaction, CrSBA-15(20)
as shown in Table 1, but the CrMCM-41(40) has higher BP
O
selectivity than other CrMCM-41 [9]. Although cobalt-containing
mesoporous catalysts gives BP O selectivity as reported in the
literature [29], they give less than 40% of DPM selectivity, but the
CrSBA-15(8) catalyst give higher DPM conversion than cobalt-
containing mesoporous catalysts. On the basis of all catalytic
activities obtained using different Cr-containing mesoporous
catalysts along with literature, it can be importantly observed that
the CrSBA-15(8) has superior catalytic activity as compared to
other catalysts.