Selective aerobic oxidation of activated alkanes with MOFs and their use for epoxidation of olefins with oxygen in a tandem reaction

Article abstract of DOI:10.1039/c2cy20449e

MOFs with Cu²⁺ centers linked to four nitrogen atoms from azaheterocyclic compounds, i.e., pyrimidine [Cu(2-pymo)₂] and imidazole [Cu(im)₂], are active catalysts for aerobic oxidation of activated alkanes, such as tetralin, cumene and ethylbenzene. Differences in activity among the two MOFs appear to be related to differences in their ability to decompose the hydroperoxide and to coordinate to the resulting radical OH species. Copper ions in [Cu(im)₂] can coordinate by expanding their coordination sphere from 4 to 5 in a reversible way, while in the case of [Cu(2-pymo)₂] it results in a displacement of one of the pyrimidine ligands. The MOFs can be used in combination with a silylated Ti-MCM-41 to catalyze the epoxidation of olefins with oxygen by means of a tandem reaction in which the MOF produces cumene hydroperoxide, which is used by Ti-MCM-41 to epoxidize the olefin. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20449e

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Selective aerobic oxidation of activated alkanes with
MOFs and their use for epoxidation of olefins with
oxygen in a tandem reaction
Cite this: Catal. Sci. Technol., 2013,
, 371
3
I. Luz, A. Le o´ n, M. Boronat, F. X. Llabr e´ s i Xamena* and A. Corma*
2+
MOFs with Cu centers linked to four nitrogen atoms from azaheterocyclic compounds, i.e., pyrimidine
Cu(2-pymo) ] and imidazole [Cu(im) ], are active catalysts for aerobic oxidation of activated alkanes,
[
2
2
such as tetralin, cumene and ethylbenzene. Differences in activity among the two MOFs appear to be
related to differences in their ability to decompose the hydroperoxide and to coordinate to the
ꢀ
Received 29th June 2012,
resulting radical OH species. Copper ions in [Cu(im)
2
] can coordinate by expanding their coordination
] it results in a displacement of
Accepted 29th August 2012
sphere from 4 to 5 in a reversible way, while in the case of [Cu(2-pymo)
2
one of the pyrimidine ligands. The MOFs can be used in combination with a silylated Ti-MCM-41 to
catalyze the epoxidation of olefins with oxygen by means of a tandem reaction in which the MOF
produces cumene hydroperoxide, which is used by Ti-MCM-41 to epoxidize the olefin.
DOI: 10.1039/c2cy20449e
www.rsc.org/catalysis
conditions studied, a relatively high T–H conversion (ca. 52%)
was attained without affecting the crystalline structure of the
1. Introduction
There is much interest in the design of hybrid materials for solid, which allowed to reuse the MOF without important loss
1
9
selective and multi-step catalytic processes. In this field, of activity. However, this Cu-MOF showed some limitations.
Metal–Organic Frameworks (MOFs) can play an important role The most important was the high level of tetralinhydroperoxide
due to the possibility to control and fine tune the crystalline (T–OOH) accumulated at the beginning of the reaction (up to
structure, porosity, chemical environment and functionalities 24 mol% yield) and a low selectivity to the ketone, a-tetralone
of the material. Compared to purely inorganic porous materials, (TQO). We found that this inconvenience can be avoided by
MOFs offer in general higher flexibility for the design of the catalytic physically mixing [Cu(2-pymo) ] with another MOF, i.e., cobalt
2
2–4
11
center,
geneous catalysis, provided that mild reaction conditions are
required. It is possible to design MOF-based heterogeneous catalysts aerobic oxidation of tetralin to a copper-containing MOF,
which makes them interesting materials for hetero- benzimidazole ZIF-9, that decomposes the hydroperoxides.
In the present work, we have extended our studies on the
1
2
in which the active sites are located either at the metallic sites or at [Cu(im) ] (im = imidazolate ), which has a chemical composi-
2
the organic linkers, or even associated to catalytic species bonded or tion and crystalline structure related to that of [Cu(2-pymo)2].
5,6
entrapped inside the pore system of the MOF. Moreover, this Recently, we have successfully used both materials for coupling
1
3
large flexibility can readily lead to the design of multi-functional reactions; viz. 1,3-dipolar cycloadditions of azides to alkynes,
7,8
MOF catalysts by combining two or more types of active centers in and three-component coupling of aldehydes, amines and term-
1
4
the same material, such as metal–metal, metal–acid, metal–base or inal alkynes. In the present work, we have found that
acid–base functionalities.
[Cu(im) ] has a better catalytic performance as compared
We have recently reported that a copper-containing MOF, to [Cu(2-pymo) ] for tetralin oxidation, in terms of activity,
2
9
2
1
0
[
2
Cu(2-pymo) ] (2-pymo = 2-oxypyrimidinolate ), could be of maximum T–H conversion and selectivity to the ketone. At
interest as heterogeneous catalyst for the liquid-phase oxida- the same time, [Cu(im) ] produces less T–OOH accumulation
2
tion of tetralin (T–H) using air as oxidant. Under the reaction in the reaction medium than [Cu(2-pymo) ] under the same
2
conditions, avoiding the use of a second MOF to deal with the
hydroperoxides. Therefore, [Cu(im) ] largely overcomes the
limitations of the previously reported [Cu(2-pymo) ] as a
2
catalyst for aerobic tetralin oxidation, without any need to
prepare physical mixtures with a second catalyst. Besides
2
Instituto de Tecnolog ´ı a Qu ´ı mica UPV-CSIC, Universidad Polit ´e cnica de Valencia,
Consejo Superior de Investigaciones Cient ´ı ficas, Avda. de los Naranjos s/n, 46022
Valencia, Spain. E-mail: fllabres@itq.upv.es, acorma@itq.upv.es;
Fax: +34 963877809
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tetralin, we have also studied the aerobic oxidation of other kept fixed to model the partial flexibility of the MOF material,
substrates; viz., cumene and ethylbenzene, which let us make and (ii) a full optimization in which the positions of all atoms in
an evaluation of the potential of both [Cu(2-pymo) ] and the system were allowed to relax without any restriction. Calcula-
2
[
Cu(im) ] as catalysts for the oxidation of benzilic paraffins. tions were performed with different spin states of the central
2
Since both MOFs were active for such oxidations we have copper atom for all models, and the singlet state was found the
attempted a cascade reaction. Then, we will also show that, most stable in all cases. Finally, thermal corrected Gibbs free
when [Cu(2-pymo)
2
] is combined with a second solid catalyst energies at 298 K were obtained from vibrational frequency
(silylated Ti-MCM-41), the in situ generated hydroperoxide can be calculations at the same B3PW91/6-311G(d,p) level of theory.
used to transform an olefin into the corresponding epoxide with
good selectivity and acceptable yield. A comparison will be made
between the performance of this mixed catalytic system under
different working setups (i.e., one-pot and two-pot setups).
2
.4. Catalytic reactions
Liquid phase oxidation. Liquid phase aerobic oxidation of
tetralin, cumene and ethylbenzene was carried out in two-
necked flask reactors with the amount of catalysts to obtain
an alkane/metal molar ratio of 2000 (for tetralin) or 200 (for
cumene and ethylbenzene). The system was heated at either
2. Materials and methods
2.1. Preparation of the Cu-MOFs
90 1C (for tetralin) or 80 1C (for cumene and ethylbenzene) and
The Cu-MOFs were prepared according to the corresponding air was fed into the reactor by bubbling at a constant rate
1
0
À1
procedures reported in the original references for [Cu(2-pymo) ]
(0.5 ml s ) by means of an adjustable valve. A reflux system
2
12
and [Cu(im)2]. X-ray diffraction (Phillips X’Pert, Cu Ka radiation) with PEG at 0 1C was installed, so that vapours were condensed,
was used to confirm the expected crystalline structure of the the total mass balances being 99 Æ 1%. After each catalytic test,
materials.
the solid catalyst was recovered by filtration and thoroughly
washed with acetone and dried in an oven at 60 1C overnight.
The crystallinity of the used catalyst was confirmed by X-ray
2.2. Silylated Ti-MCM-41
Silylated Ti-MCM-41 was prepared following the original diffraction, and compared with the fresh material. Time evolu-
1
5
procedure. First, Ti-MCM-41 was obtained from a gel having tion of products was determined by GC analysis of the samples
the following molar composition: SiO : 0.015 Ti(OEt)
: 0.26 (Varian 3900, capillary column HP-5) using n-hexadecane as an
CTABr : 0.26 TMAOH : 24.3 H
O where CTABr is cetyltrimethyl- external standard. The intermediate hydroperoxides formed
2
4
2
ammonium bromide and TMAOH is tetramethylammonium (T–OOH, CM–OOH and EB–OOH) were quantitatively deter-
hydroxide. The silica source, Aerosil-200, was obtained from mined by means of an indirect method using triphenyl-
2
0
Degussa. The crystallization was performed at 100 1C for 48 h in phosphine (TPP). Preliminary experiments were performed to
Teflon lined stainless steel autoclaves. The occluded surfactant find the conditions under which the reaction is not controlled
was completely removed following a two step extraction by external diffusion. Then, three experiments were performed
procedure. Ti-MCM-41 was then fully silylated with hexamethyl- at stirring rates of 500, 1000 and 1500 rpm. No differences in
disilazane (HMDS) as the silylating agent. The silylation was the initial reaction rate were observed, indicating that for
carried out at 120 1C with a solution of HMDS in toluene under stirring speeds of 500 rpm the reaction is not controlled by
inert atmosphere. The MCM-41 structure was preserved after external diffusion. Therefore, the experiments have always been
silylation and the surface area of the silylated sample was close performed at 500 rpm.
2
À1
to 1000 m g
.
Tandem cumene oxidation and 1-octene epoxidation. 1 ml
of cumene and 1 ml of 1-octene were placed in a 10 ml pressured
2
.3. Computational details
tubular reactor. A solid mixture of 5 mg of [Cu(2-pymo) ] and
2
1
5
Calculations were carried out by means of the Gaussian03 2.5 mg of sylilated Ti-MCM-41 (2.1 wt% TiO
2
)
were added to the
1
6
program package
method
using the density functional B3PW91 solution and the reactor was pressured at 3 bar with air. The
and the standard 6-311G(d,p) basis set. The Cu reaction was carried out at two control temperatures: 333 K
1
7,18
19
active sites in [Cu(im) ] and [Cu(2-pymo) ] MOFs were modeled and 363 K.
2
2
2
+
by means of two clusters of atoms consisting of a central Cu
cation surrounded by either four imidazole or four 2-hydroxy-
pyrimidine molecules. Each of the four organic ligands in the
resulting models was saturated with a proton placed in the
direction of the next Cu cation in the real MOF material, so Oxidation of tetralin to the corresponding ketone, a-tetralone,
that each model bears a net positive charge of +2, and the is interesting for the preparation of a number of chemicals
geometry of the two cluster models was fully optimized without (insecticides, agrochemicals and drugs) and for the production
3
. Results and discussion
3.1. Liquid phase oxidation of tetralin
2
+
21
any restriction. For the complexes formed by interaction of the of oxygenated diesel molecules. This reaction can be performed
2
+
ꢀ
ꢀ
Cu sites with CM–OOH, with CM–O radical and with a OH using air as the oxidant and in the presence of a suitable
radical, two types of geometry optimizations were performed: oxidation catalyst, yielding a mixture of tetralinhydroperoxide
(i) a restricted optimization, in which the positions of the four (T–OOH), a-tetralol (T–OH) and a-tetralone (TQO), as depicted
terminal hydrogen atoms saturating the organic ligands were in Scheme 1:
3
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a higher activity than [Cu(2-pymo)
2
], with turnover frequencies
(
TOF) calculated from the initial reaction rates from Fig. 1
À1
being 19 000 and 4000 h for [Cu(im) ] and [Cu(2-pymo) ],
2
2
respectively. Thus, for instance, after 8 h of reaction, the
conversion of T–H over [Cu(2-pymo) ] was only 18%, while
2
Scheme 1 Tetralin (T–H) oxidation reaction, leading to a-tetralinhydroperoxide
the conversion over [Cu(im)
experiments performed in the absence of any MOF catalyst
i.e., autothermal oxidation) gave very poor conversions, attaining a
2
] already reached 55%. Blank
(
T–OOH), a-tetralol (T–OH) and a-tetralone (TQO).
(
maximum T–H conversion of 1.3 mol% after 24 h (see Table 1),
T–OOH being the only product formed.
Another important parameter of the oxidation reaction is
the final selectivity to the ketone, which is usually expressed as
the molar ratio of a-tetralone to a-tetralol, TQO/T–OH. As it can
be seen in Table 1, also from this point of view the performance
of [Cu(im)
T–OH calculated at maximum conversion were 3.5 and 2.7
for [Cu(im) ] and [Cu)2-pymo) ], respectively. Finally, as we
mentioned above, the use of [Cu(2-pymo) ] as catalyst for
2 2
] is better than [Cu(2-pymo) ]: the values of TQO/
2
2
2
T–H oxidation showed an undesired accumulation of hydro-
peroxides in the reaction medium, which reaches up to 24% of
the initial amount of T–H after 18 h. The presence of large
quantities of the hydroperoxide during the reaction can be a
safety hazard, so must be avoided. On the contrary, when
Fig. 1 Conversion of T–H over [Cu(im)
2
] and [Cu(2-pymo)
2
] (part a). Time
] and [Cu(2-
2
[Cu(im) ] was used as catalyst, the concentration of T–OOH
conversion of T–H and time evolution of products over [Cu(im)
2
was kept much lower throughout the reaction, reaching a
maximum concentration of only 9 mol% at short reaction
time (1 h) and dropping below 1–2 mol% shortly afterwards
pymo)
2
] is also shown in parts (b) and (c), respectively.
9
As we have shown in a previous work, the copper-containing (see Fig. 1b).
MOF [Cu(2-pymo)
liquid phase oxidation of T–H using air as the oxidant. A use of [Cu(im)
maximum T–H conversion of about 52% was achieved after oxidation of T–H presents a number of benefits with respect
8 h of reaction at 90 1C, with a T–H/metal molar ratio of 2000. to [Cu(2-pymo) ], which makes it a very interesting material for
Conversely, when the reaction was carried out in the presence this reaction. We also observed that both catalysts were stable
of [Cu(im) ], keeping all the other reaction conditions under the experimental conditions used, since the materials
2
] is an active and reusable catalyst for the
In summary, as we have shown in Fig. 1 and Table 1, the
2
] as catalyst for the liquid phase aerobic
4
2
2
unchanged, the maximum amount of T–H converted increased recovered after the catalytic reaction showed XRD patterns
up to 68%, and this value was reached after only 22 h of practically undistinguishable from those of the fresh materials.
reaction. A comparison of the two catalysts for the oxidation The occurrence of copper leaching from the framework was
2
of T–H can be seen in Fig. 1 and Table 1. [Cu(im) ] also showed also ruled out by a hot filtration test and by chemical analysis of
2 2
Table 1 Selected kinetic data for the aerobic oxidation of various organic substrates over [Cu(2-pymo) ] and [Cu(im) ] MOF catalysts
Tetralin (T–H)
Conv. 8 h
Conv. max
TQO/T–OH
[T–OOH]max
[
[
Cu(im)
Cu(2-pymo)
2
]
55%
18%
68% (22 h)
52% (48 h)
3.5
2.7
9% (1 h)
24% (18 h)
2
]
Blank:
4 h: 0.2% T–OOH; 24 h: 1.3% T–OOH
Cumene (CM)
Conv. Max
Select. CM–OH
[CM–OOH]max
[
[
Cu(im)
Cu(2-pymo)
2
]
99% (23 h)
87% (23 h)
74%
64%
2 (1.5 h)
24 (8 h)
2
]
Blank:
4 h: 0.5% CM–OOH; 24 h: 21% CM–OOH + 3% AP
Ethylbenzene (EB)
Conv. 5 h
Conv. max
Select. AP
[
[
Cu(im)
Cu(2-pymo)
2
]
23%
6%
29% (40 h)
25% (40 h)
86%
87%
2
]
Blank:
4 h: No conversion; 24 h: 2% EB–OH + 0.5% EB–OOH
T–H: tetralin; T–OOH: tetralinhydroperoxide; TQO: a-tetralone; T–OH: a-tetralol; CM: cumene; CM–OOH: cumene hydroperoxide (2-phenyl-2-
propylhydroperoxide); CM–OH: cumene alcohol (2-phenyl-2-propanol); EB: ethylbenzene; AP: acetophenone.
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Fig. 2 Conversion of CM over [Cu(im)
2
] and [Cu(2-pymo)
2
] (part a). Time
] and [Cu(2-
Fig. 3 Conversion of EB over [Cu(im)
2
] and [Cu(2-pymo)
2
] (part a). Time con-
conversion of CM and time evolution of products over [Cu(im)
pymo) ] is also shown in parts (b) and (c), respectively.
2
version of EB and time evolution of products over [Cu(im)
also shown in parts (b) and (c), respectively.
2
] and [Cu(2-pymo) ] is
2
2
the filtrate after removing the solid catalyst at the end of the (25% and 23%, respectively). It can also be observed in Fig. 2
reaction. Stability and leaching tests were already reported and that the amount of accumulated CM–OOH is higher (and lasts
9
discussed in detail for [Cu(2-pymo)
2
] in our previous work, so longer) in the case of [Cu(2-pymo)
2
] (up to 24% CM–OOH after
they are not reproduced again here, the results obtained with 8 h, see Table 1). However, the activity of both materials is
Cu(im) ] being completely analogous. practically the same at short reaction times (up to 2 h), as
Encouraged by the good results obtained with [Cu(im) ] for evidenced by the superposition of both curves in Fig. 2a. From
[
2
2
T–H oxidation, we extended our studies to other substrates: the slopes of the time-conversion curves at short reaction times,
À1
cumene and ethylbenzene. We wanted on the one hand to a TOF of about 3500 h
was calculated for both copper
determine the general applicability of these copper-containing catalysts. A blank experiment in the absence of catalyst yielded
MOFs as catalysts for paraffin oxidation. And if the result were 21% CM–OOH and 3% AP after 24 h (see Table 1).
positive, we thought of designing a multistep catalytic process
in which the cumene hydroperoxide formed could further react,
as oxidizing agent, for the synthesis of epoxides from olefins.
3.3. Ethylbenzene oxidation
Oxidation of ethylbenze is expected to be more difficult than in
the case of tetralin or cumene, since the carbon atom to be
oxidized is less activated. In the case of ethylbenzene, oxygen is
3.2. Liquid phase oxidation of cumene
Oxidation of cumene (CM) with air yields cumene hydroperoxide added onto a secondary carbon atom, while in cumene the
CM–OOH) in the first place, which is then converted into cumene preferred position for the attack is on a tertiary carbon atom,
alcohol (CM–OH), usually accompanied by other byproducts which allows a major stabilization of the radical species
mainly acetophenone, AP) coming from non-selective decomposi- formed. In tetralin the addition of oxygen is onto a secondary
(
(
22
tion side reactions. Fig. 2 and Table 1 show a comparison of the carbon atom, but in this case, it belongs to a six-member ring,
results obtained for the liquid phase aerobic oxidation of cumene which adds an extra activation as compared to ethylbenzene.
(
80 1C, 0.5 mol% Cu) using either [Cu(2-pymo)
2
] or [Cu(im)
2
] as
Although the conversion of ethylbenzene was considerably
lower than that observed for cumene under similar conditions
catalyst.
The results obtained for cumene oxidation are qualitatively (at 80 1C, 0.5 mol% Cu), both [Cu(2-pymo) ] and [Cu(im) ]
2
2
very similar to those obtained for the oxidation of T–H, which demonstrated to be active catalysts for this reaction, as shown
demonstrates that both Cu-MOFs are able to catalyze this in Fig. 3. In this case, [Cu(im) ] showed a better performance at
2
reaction. As occurred before for T–H, the imidazolate material short reaction time, although at the end of the reaction the two
[
Cu(im)
pared to [Cu(2-pymo)
between the two catalysts are less pronounced than in the [Cu(2-pymo)
oxidation of T–H. Complete CM conversion was achieved after If we compare the kinetic data obtained for EB oxidation
3 h over [Cu(im) ], while some lower conversion (i.e., 87%) was (Fig. 3) with the data obtained for TH and CM oxidation (Fig. 1
2
] also shows a better oxidation performance as com- materials gave similar results: 29% (25%) conversion after 40 h
], although the differences observed and 86% (87%) selectivity to acetophenone for [Cu(im) ] and for
], respectively.
2
2
2
2
2
obtained with [Cu(2-pymo) ]. The selectivity towards CM–OH and 2), clear differences can be observed. On the one hand,
2
was also higher in the case of [Cu(im) ] (74%, versus 64% accumulation of hydroperoxide in the reaction medium does
2
2 2
obtained for [Cu(2-pymo) ]). In both cases, at the end of not occur during EB oxidation, not even when [Cu(2-pymo) ]
the reaction the sole product observed besides CM–OH was was used as catalyst. This is not surprising, since the hydro-
acetophenone, which accounted for the rest of CM converted peroxide derived from ethylbenzene is the least stable among
3
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2
all the substrates studied in this work. Thus, when 1-phenyl- [Cu(2-pymo) ], while they are in a highly distorted coordination in
2
hydroperoxide (EB–OOH) is generated, it is immediately converted [Cu(im) ], with trans N–Cu–N angles of 1401 and 1551. In a first
into other products (i.e., acetophenone and 1-phenylethanol). On approximation, one can expect that these structural differences
the other hand, we also observed clear induction periods of could account for the different reactivity observed for both
about 2–3 h, in which EB is not converted, irrespective of the materials. However, it is difficult for us to explain intuitively the
2+
catalyst used. Both observations indicate that in the EB oxidation structure–composition–reactivity correlation for the two Cu MOFs
reaction, formation of the hydroperoxide species is difficult for studied. Then, and in order to get more insights into the structural
both Cu-MOF catalysts. However, once this primary species is and electronic properties of the two compounds, we have carried
formed, it is converted to reaction products more effectively over out first principle DFT calculations on MOF model clusters, as
[
Cu(im)
2 2
] than over [Cu(2-pymo) ]. A blank experiment in the described in detail in the Experimental section. The models used
2+
absence of catalyst yielded only 2% EB–OH and 0.5% EB–OOH consisted of a central Cu ion surrounded by either four imidazole
after 24 h (see Table 1).
(im) or four 2-hydroxypyrimidine (2-pymo) molecules. Each of the
four organic ligands in the cluster models was saturated with a
proton placed in the direction of the next Cu ion in the real MOF
3.4. Origin of the different reactivities of [Cu(2-pymo)
2
] and
2+
[Cu(im)
2
]
material, so that each model bears a net positive charge of +2. The
In light of the results presented above, and which are summarized geometry of the two resulting clusters was fully optimized without
in Table 1, it is evident that [Cu(im) ] is a better catalyst than restrictions.
2
[
Cu(2-pymo)
have observed that both formation of the primary hydroperoxide two copper-containing MOFs [Cu(im)
species and its conversion into final products is faster for have theoretically investigated using DFT the interaction of
2
] for the oxidation of TH, CM and EB. In general, we
To shed some light on the different catalytic behavior of the
] and [Cu(2-pymo) ], we
2
2
2+
the imidazolate compound. Both compounds feature Cu ions these two materials with cumene-hydroperoxide (CM–OOH) to
2
+
coordinated to four N atoms from different diazaheteorcyclic form an adsorption complex [Cu –HOO–CM], and its subse-
compounds (imidazole and pyrimidine), thus leading to CuN
quent dissociation into a hydroxyl radical that remains
4
2+
2+
centers. However, Cu ions are in square planar coordination in adsorbed on the copper center [Cu –OH] and a free cumyl
2 2
Fig. 4 Optimized structures of the complexes formed by interaction of: (a) cumene-hydroperoxide with [Cu(im) ] (b) cumene-hydroperoxide with [Cu(2-pymo) ] (c)
hydroxyl radical with [Cu(im)
and yellow, respectively.
2
] and (d) cumene-hydroperoxyl radical and [Cu(im)
2
]. Carbon, nitrogen, oxygen, hydrogen and copper atoms are grey, blue, red, white
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ꢀ
2+
(
O–CM) radical. The fully optimized structures (see Computa- displaced can coordinate again to the Cu site to recover the
tional details) resulting from CM-OOH adsorption on the initial catalytic centre, as shown in Scheme 2. A similar ligand
2
+
two Cu active site models are depicted in Fig. 4. CM-OOH displacement and re-coordination cycle has been demonstrated
adsorption was found to be energetically favorable on the to occur in a series of zinc(II) benzoate coordination polymers
2
3
two materials studied, with calculated free energy values of during transesterification reactions.
À1
À1
À6 kcal mol for [Cu(im)
2
] and À2 kcal mol for [Cu(2-pymo)
2
],
In conclusion, the results obtained from first principle
] has a more adaptable
], which allows the
irrespectively of the type of geometry optimization performed calculations indicate that [Cu(im)
2
2
+
(
full or restricted). The optimized Cu –O distances found were crystalline framework than [Cu(2-pymo)
2
2
.47 and 2.30 Å in [Cu(im)
2
] and [Cu(2-pymo)
2
] materials, copper sites to expand their coordination sphere from 4 to
ꢀ
respectively, also indicating a certain degree of interaction. It 5 upon interaction with OH radical species. On the contrary,
is worth noting that the initial geometry of the active center binding of the same radical to [Cu(2-pymo) ] produces the
2
in both MOFs is not significantly distorted in any case as a displacement of one of the 2-pymo ligands from the coordina-
consequence of the interaction with the hydroperoxide. How- tion sphere around the central Cu site. These differences could
ever, after cumene hydroperoxide dissociation, substantial account for the higher activity of [Cu(im)
2
] to decompose the
2
+
differences were found in the resulting [Cu -OH] complexes hydroperoxide into final products, and at the same time could
2
+
formed in the two materials. On the one hand, Cu ions in explain the experimentally observed accumulation of the
[
4
Cu(im)
2
] were found to expand their coordination sphere from CM–OOH intermediate in the oxidation of CM catalyzed by
ꢀ
to 5 upon binding of a OH radical. On the contrary, [Cu(2-pymo)
2
]. Even if this preliminary computational
ꢀ
2+
the interaction of the OH radical with the Cu centre in study does not include calculation of the transition states
Cu(2-pymo) ] implies the de-coordination of one of the four corresponding to the two Cu-MOFs, it is not unreasonable to
[
2
2-pymo ligands. Note that this ligand displacement would not expect that a higher energy would be required in the case of
necessary imply the collapse of the crystalline structure of the [Cu(2-pymo) ] to break a Cu–pyrimidine bond that in the case
2
2
+
MOF. Actually, the Cu centers would still remain connected to of [Cu(im)
2
] in which only a rearrangement of the ligands is
ꢀ
the framework through three out of the four initial 2-pymo required to accommodate the OH radical.
ligands. Once the catalytic cycle is finished and the product
Nevertheless, in order to demonstrate that the two MOFs have
desorbs from the active site, the 2-pymo ligand that has been indeed different abilities for decomposing the hydroperoxide, we
Scheme 2 Ligand displacement and re-coordination of 2-hydroxypyrimidine in [Cu(2-pymo)
2
] upon coordination/release of a radical species.
3
76 Catal. Sci. Technol., 2013, 3, 371--379
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amount of hydroperoxide is accumulated in the reaction med-
ium, especially in the case of cumene and tetralin oxidation.
This limitation can be turned into an advantage if we are able to
use the hydroperoxide generated in situ during the oxidation as
a reagent for a second reaction. This becomes even more
interesting if one can couple the two reactions in a one-pot
process, thus leading to a tandem catalytic process. Taking as a
2
4,25
source of inspiration the Sumitomo process
epoxidation using cumene hydroperoxide, we have combined
Cu(2-pymo) with state-of-the-art epoxidation catalyst
silylated Ti-MCM-41 ) to perform the tandem process
depicted in Scheme 3. The experimental setup is shown in
for olefin
Scheme 3
[
(
2
]
a
1
5
have designed an additional experiment. Thus, when we con- Fig. 5a. Thus, [Cu(2-pymo) ] is used in the first reaction to
2
tacted the two MOFs with cumene hydroperoxide, it was oxidize CM into CM–OOH using air as the oxidant (reaction
observed that [Cu(im) ] decomposes the hydroperoxide signifi- R1). Then, silylated Ti-MCM-41 can use the generated CM–OOH
2
cantly faster than [Cu(2-pymo) ] under identical conditions to oxidize an olefin (1-octene) in reaction R2, producing
2
(
0.01 mmol Cu, 0.9 ml CM, 0.1 ml CM–OOH, 80 1C under a N
2
1-octene oxide and one equivalent of CM–OH.
atmosphere). After 1 hour, [Cu(im) ] decomposed 55 mol% of
2
However, this simple reaction scheme is complicated by the
the initial CM–OOH, while only 31 mol% was decomposed over occurrence of competing unwanted side reactions, which
[
Cu(2-pymo)
2
].
decrease the overall selectivity to 1-octene oxide by either
yielding secondary products of 1-octene (SR2 and/or SR3), or
by spuriously consuming CM–OOH without transferring the
oxygen to 1-octene (SR1). Side reaction SR1 consists in the
3
.5. Tandem cumene oxidation and 1-octene epoxidation
using a combination of catalysts: one-pot versus two-pot setups
As we have mentioned above, one of the drawbacks of decomposition of the CM–OOH into CM–OH and AP. This
Cu(2-pymo)
] as oxidation catalyst is that a considerable reaction is catalyzed by [Cu(2-pymo) ], as we have already
[
2
2
Fig. 5 Schematic representation of the one-pot (a) and two-pot setups (b) adopted for the tandem cumene oxidation/1-octene epoxidation reaction. CM = cumene;
CM–OOH = cumene hydroperoxide; AP = acetophenone; Epox = 1-octene oxide.
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Catalysis Science & Technology
Table 2 Summary of the results obtained for the one-pot tandem reaction 1-octene oxide (as well as the presence of acetophenone in
shown in Scheme 3
the products mixture) indicates that CM-OOH has been spur-
iously decomposed without producing the desired epoxide.
Temp Conv./Time
Yield epox Yield oxdn. Select.
CM–OH/
(
K)
(mol%/h)
(mol%)
(mol%)
epox (%) epox
This is an important factor to take into account, since in the
envisaged reaction scheme the CM-OH produced has to be
recovered and reconverted into CM (through a dehydration and
hydrogenation process) for recycling. If we compare our results
obtained at 333 and 363 K, we can see that at the lower
temperature, although the reaction rate is considerably low,
the process is highly selective concerning the utilization of the
3
3
33
63
7.1 (48)
17.1 (24)
4.5
10.8
2.6
6.3
63.4
63.2
1.1
3.0
epox = 1-octene oxide; oxdn = products coming from the allylic oxida-
tion of 1-octene.
demonstrated in the previous section, and it can also take place CM-OOH formed (CM-OH/epoxide = 1.1). On the contrary, upon
thermically (non-catalytic decomposition). SR2 consists in the increasing the temperature of the reaction, the alternative SR1
direct allylic oxidation/isomerization of 1-octene, to produce side reaction becomes the main reaction pathway for CM-OOH
a mixture of oxidized products. SR2 can also be catalyzed decomposition (CM-OH/epoxide = 3.0). Thus, an increase of
by copper or an uncatalyzed thermal reaction. Finally SR3, the reaction temperature has negative effects on the amount of
consisting in the ring opening of the epoxide, is another CM-OH waste generated per mol of final product.
reaction that in principle could decrease the overall yield of
-octene oxide.
One evident limitation of this one-pot setup is the relatively
low selectivity to the epoxide (63.2% at 363 K), which is due,
1
After a preliminary screening of different conditions, we as mentioned above, to the occurrence of the competing
found a satisfactory experimental setup, as detailed in the side-reaction SR2: allylic oxidation of 1-octene catalyzed by
experimental section. Among others, the following parameters [Cu(2-pymo) ]. The only way of suppressing this reaction, thus
2
were considered: (i) [Cu(2-pymo)
(
(
2
] to silylated Ti-MCM-41 ratio; increasing the selectivity to the epoxide product, is to avoid the
ii) cumene to 1-octene ratio; (iii) temperature of the reaction; contact between 1-octene and the copper-MOF by keeping them
iv) order of addition of the reagents; and (v) oxygen pressure. in separate reactors. In other words, it becomes necessary to
To evaluate the relevance of the different competing side adopt a two-pot setup, such as that shown in Fig. 5b. In such a
reactions, we carried out the oxidation process at two different reaction setup, [Cu(2-pymo) ] is allowed to interact with CM
temperatures, viz. 333 and 363 K. A summary of the results under an O atmosphere at 363 K for 4 h, to produce CM-OOH
2
2
obtained is shown in Table 2.
in 24 mol% yield (see Fig. 5b). Then, the copper-catalyst is
When the reaction was performed at 333 K, an overall removed by filtration and the liquid filtrate is fed into a second
conversion of 1-octene of 7.1 mol% was achieved after 48 h, reactor, containing silylated Ti-MCM-41, and 1-octene is added
producing 1-octene oxide with a selectivity of 63.4%, together at this point. Under these conditions, 1-octene was selectively
with other products coming from the allylic oxidation of converted into the corresponding epoxide, yielding 18.1 mol%
1
[
1
-octene. In a separate experiment we have observed that pure after 24 h at 363 K. In this case, no traces of products coming
Cu(2-pymo) ] can indeed catalyze the allylic oxidation of from either the allylic oxidation of 1-octene, or from the ring-
-octene using air as the oxidant, yielding mainly the products opening of the epoxide, were detected. A final CM-OH/epoxide
2
indicated in Scheme 3 (12% maximum conversion of 1-octene ratio of 3.0 was obtained. Note that most of this CM-OH was
achieved after 24 h at 363 K). Note that these oxidation products already present in the filtrate coming from the first reactor
can also be formed through a non-catalyzed thermal reaction. It (38 mol%, see Fig. 5b). Indeed, CM-OOH is used to oxidize the
is important to stress that in our system we did not detect any olefin in the second reactor with a high selectivity (24 mol%
traces of products coming from the ring aperture of the formed CM-OOH produces 18.1 mol% of 1-octene oxide), while direct
epoxide (SR3), which is largely suppressed by the hydrophobic decomposition of the hydroperoxide occurs only to a minor
character of the silylated Ti-MCM-41 catalyst used and the extent in the second reactor. This is largely prevented by the
1
5
absence of water in the reaction medium.
absence of the copper catalyst at this stage (which would
When the reaction temperature was increased to 363 K, the catalyze the side reaction SR1) and due to the relatively low
overall reaction rate dramatically increased: 17.1% 1-octene temperature (which minimizes the autocatalytic thermal
conversion after 24 h was observed, which represents an almost decomposition of CM-OOH).
5-fold increase of the reaction rate. Interestingly, the selectivity
to 1-octene oxide was practically the same (63.2%), with a
similar distribution of secondary products than at lower
temperature.
Another important parameter of the tandem reaction is which Cu centers are linked to 4 nitrogen atoms from
given by the CM-OH/epoxide ratio, which reveals the extent to azaheterocyclic compounds (i.e., pyrimidine and imidazole)
which the competing side reaction SR1 is taking place. Indeed, have interesting potential as heterogeneous catalysts for aero-
if all the CM-OOH formed in R1 is used to produce 1-octene bic liquid phase oxidation of activated paraffins. We have
4
. Conclusion
In this work, we have shown that copper-containing MOFs in
2
+
oxide following R2, the final CM-OH/epoxide ratio should be shown this for two materials, [Cu(im)
2 2
] and [Cu(2-pymo) ],
equal to 1. Any excess amount of CM-OH with respect to and for three different substrates of increasing demands:
3
78 Catal. Sci. Technol., 2013, 3, 371--379
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tetralin, cumene and ethylbenzene. Our results indicate that
4 J. Y. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen
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[Cu(im) ] has in general better performance than [Cu(2-
pymo) ], which results in higher alkane conversion, higher
2
selectivity and low accumulation of alkylhydroperoxides in
the reaction medium. According to DFT calculations, the
differences between the two catalysts could be related to the
different ability of the two MOFs to decompose the hydro-
ꢀ
peroxide and to coordinate to the resulting radical OH species.
2
Copper ions in [Cu(im) ] can coordinate to these radicals by
expanding their coordination sphere from 4 to 5. On the
8 F. G. Cirujano, F. X. Llabr ´e s i Xamena and A. Corma, Dalton
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2
2
+
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not in the case of [Cu(im) ].
We have also shown that the hydroperoxide accumulated in
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be used as oxidant in a parallel reaction. To illustrate this, we
2
] is used as catalyst, and
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2
2
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2
(
silylated Ti-MCM-41) to carry out a tandem process consisting
in cumene oxidation and 1-octene epoxidation. We have studied 13 I. Luz, F. X. Llabr ´e s i Xamena and A. Corma, J. Catal., 2010,
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2
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becomes necessary to work in two batch reactors.
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Acknowledgements
Financial support by Ministerio de Educaci ´o n y Ciencia e
Innovaci ´o n (Project MIYCIN, CSD2009-00050; PROGRAMA CON- 18 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
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TEO/2008/130) and CSIC (Proyectos Intramurales Especiales
213–222.
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01080I020) is gratefully acknowledged. CSIC and Fundaci o´ n 20 M. M. Fares, M. El-Khateeb and K. J. Asali, J. Inorg. Organo-
Bancaja are gratefully acknowledged for a research contract to
I.L. AL thanks the ITQ for a postgraduate scholarship.
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21 G. Y. Tian, D. H. Xia and F. T. Zhan, Energy Fuels, 2004, 18, 49–53.
22 C. Aprile, A. Corma, M. E. Domine, H. Garcia and
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