Diels-Alder reactions in confined spaces: The influence of catalyst structure and the nature of active sites for the retro-Diels-Alder reaction

Article abstract of DOI:10.3762/bjoc.12.208

Diels-Alder cycloaddition between cyclopentadiene and p-benzoquinone has been studied in the confined space of a pure silica zeolite Beta and the impact on reaction rate due to the concentration effect within the pore and diffusion limitations are discussed. Introduction of Lewis or Br?nsted acid sites on the walls of the zeolite strongly increases the reaction rate. However, contrary to what occurs with mesoporous molecular sieves (MCM-41), Beta zeolite does not catalyse the retro-Diels-Alder reaction, resulting in a highly selective catalyst for the cycloaddition reaction.

Full text of DOI:10.3762/bjoc.12.208

Diels–Alder reactions in confined spaces: the influence of
catalyst structure and the nature of active sites for the
retro-Diels–Alder reaction
Ángel Cantín1, M. Victoria Gomez*2 and Antonio de la Hoz*2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 2181–2188.
1Instituto de Tecnología Química (UPV-CSIC), Universidad
Politécnica de Valencia, Avda. Los Naranjos s/n, 46022 Valencia,
Spain and 2Área Química Orgánica, Facultad de Químicas,
Universidad de Castilla-La Mancha, and Instituto Regional de
Investigación Científica Aplicada (IRICA), Avda. Camilo José Cela
s/n, E-13071-Ciudad Real, Spain
doi:10.3762/bjoc.12.208
Received: 28 July 2016
Accepted: 27 September 2016
Published: 13 October 2016
This article is part of the Thematic Series "Green chemistry".
Guest Editor: L. Vaccaro
Email:
M. Victoria Gomez* - Mariavictoria.Gomez@uclm.es;
Antonio de la Hoz* - Antonio.Hoz@uclm.es
© 2016 Cantín et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
catalysis; Diels–Alder; retro-Diels–Alder; zeolites
Abstract
Diels–Alder cycloaddition between cyclopentadiene and p-benzoquinone has been studied in the confined space of a pure silica
zeolite Beta and the impact on reaction rate due to the concentration effect within the pore and diffusion limitations are discussed.
Introduction of Lewis or Brønsted acid sites on the walls of the zeolite strongly increases the reaction rate. However, contrary to
what occurs with mesoporous molecular sieves (MCM-41), Beta zeolite does not catalyse the retro-Diels–Alder reaction, resulting
in a highly selective catalyst for the cycloaddition reaction.
Introduction
The Diels–Alder reaction (DAR) is one of the most useful reac- microporous materials) represent an interesting approach for the
tions in organic synthesis. In order to improve the yield and to conversion of biomass feedstock into stable chemicals such as
avoid the reversibility of the reaction, homogeneous Lewis furfural derivatives, platform molecules which can be con-
acids [1-4], solid acids [5,6] as catalyst, high pressures [6-8] verted into a variety of liquid hydrocarbon fuels and fuel addi-
and/or water as a solvent [9,10] have been reported. In particu- tives [11,12]. Catalysis is considered as one of the foundational
lar, and among the most interesting environmental-friendly pillars of green chemistry. Catalysis often reduces the energy
reactions, the cycloaddition reaction occurs with high selec- requirements, permits the use of renewable feedstocks and less
tivity and atom economy. Moreover, Diels–Alder cycloaddi- toxic reagents. Moreover, in most cases yields are improved and
tions in combination with heterogeneous catalysts (i.e. doped- selectivity is enhanced or modified [13]. In this regard, hetero-
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geneous catalysis in general and zeolites in particular are enhances the conversion to the endo-anti-endo isomer as has
remarkably efficient since they permit the replacement of toxic been shown in a previous work [30]. However, MCM-41 in the
mineral acids and oxidants by easily recyclable catalysts [14].
form of aluminosilicate that contains Brønsted sites enhances
the retro-Diels–Alder reaction increasing the selectivity to the
One approach to improve yields and selectivity is the special endo-anti-exo isomer. Therefore, the framework and extra
confinement of the reactants and the presence of catalytic active framework composition of mesoporous materials and zeolite
sites, [15,16] by use of microporous materials doped with could be used to control the selectivity of the DAR of cyclopen-
metals. While pore dimensions and topology of the microp- tadiene and p-benzoquinone.
orous materials can affect the selectivity of the reaction, their
activity can be strongly limited by a slow diffusion of reactants In the present work, a series of large pore, pure silica zeolites
and products, unless microporous molecular sieves with the (in which rate enhancement can only occur by spatial confine-
appropriated pore dimensions are used as catalyst. Thus, micro- ment) and the same structures but containing framework
porous molecular sieves with optimized pore diameters and Brønsted or Lewis acid sites have been studied for the DAR be-
topologies can be of interest to catalyze DAR [17-26] in where tween cyclopentadiene and p-benzoquinone. The effects of pore
different stereoisomers could be obtained. Lewis-acid centers dimensions and catalyst composition on diffusivity and selec-
contained within the framework of zeolite beta (Zr-β, Sn-β) are tivity with respect to the retro-Diels–Alder reaction (retro-
useful catalysts in the Diels–Alder reaction for the production DAR) are discussed.
of bio-based terephthalic acid precursors, one of the monomers
for the synthesis of polyethylene terephthalate that is used for Results and Discussion
the large-scale manufacture of plastic bottles among others. The As it was described previously [30], the Diels–Alder reaction
authors do not find transport limitations within the zeolite (DAR) between cyclopentadiene and p-benzoquinone follows
framework to the rate of the reaction [27]. Interestingly, when the reactions outlined in Scheme 1.
Brønsted acid containing zeolites (Al-β) are used as catalyst,
there is a decrease in the Diels–Alder reaction selectivity [28]. As expected, the Diels–Alder cycloaddition provides the kineti-
cally controlled endo isomer that very rapidly reacts with a
The DAR of cyclopentadiene with p-benzoquinone is a well- second molecule of the diene to give again the kinetic endo-
known example of cycloadditions, and some results can be endo isomer. It is remarkable that neither the thermodynamic
found on the control of the selectivity to the different isomers. exo isomer nor the secondary exo-endo and exo-exo products
In homogeneous phase, equimolar amounts of diene and dieno- were detected under our reaction conditions. Thus, the ob-
phile afford two isomers, the endo as the major and the exo as served products, endo-endo and endo-exo are obtained in differ-
the minor product. The addition of a second equivalent of ent ratio according to the reaction conditions. This ratio can
cyclopentadiene affords mainly the endo-anti-endo product as change with the time since the retro-Diels–Alder reaction
major isomer, and the endo-anti-exo product as the minor appears as a competitive reaction. In this way, the final molecu-
isomer. While CsY zeolite enhances the selectivity to the endo- lar product can revert to the initial endo isomer, which in turn
anti-exo isomer [29], the mesoporous material MCM-41 can react again with a new cyclopentadiene molecule. This is
Scheme 1: Distribution of products in the Diels–Alder reaction between cyclopentadiene and p-benzoquinone.
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Beilstein J. Org. Chem. 2016, 12, 2181–2188.
reflected in the distribution products by a decrease of the endo-
endo isomer (kinetic control product) jointed to an increase of
the endo-exo (thermodynamic control product).
Influence of catalyst surface
We have seen that the DAR between cyclopentadiene and
p-benzoquinone takes place thermally. The effect of confine-
ment of the reactant within the pores of the catalyst can de-
crease the entropy of the activated complex [17-26,29,30] pro-
ducing not only an increase of the reaction rate but also a modi-
fication of the selectivity. To study this effect, we have firstly
carried out the reaction using a large pore Beta zeolite as cata-
lyst. Thus, Figure 1 compares conversion results obtained for all
silica Beta zeolite with that obtained during the thermal reac-
tion or using Aerosil (amorphous non porous silica, BET sur-
face area = 200 m2g−1) as potential catalyst. Practically no
Figure 1: Conversion in the DAR catalysed by silica Beta zeolites and
Aerosil.
differences were found on reaction rate nor on product distribu- reducing the crystallite size of the zeolite, indicating that there
tion when the reaction occurs on nonporous silica, with Beta is a reactant diffusion control within the pores of Beta zeolite
zeolite or even in absence of any solid. Considering that Aerosil and consequently the reaction is mainly occurring in an outer
is an amorphous solid, these results indicate that the catalytic shell of the crystals. If this is so, and since the reaction rate in-
reaction with pure silica Beta zeolite, if any, should only occur creases with the pure silica nanocrystalline Beta zeolite with an
on the catalyst surface and the porous structure has not any external surface area not too different from Aerosil silica, we
effect on the reaction. Another hypothesis to explain these can conclude that a concentration effect within the pore mouth
results is that diffusion of the products through the channels, if of the zeolite may be responsible for the reaction rate enhance-
ever formed inside, is strongly restricted and the products ment observed with pure silica nanocristalline beta.
remain adsorbed within the pores. To evaluate this second
Introduction of Lewis and Brønsted acid sites
hypothesis, 13C MAS NMR, elemental analysis and material
balance were done, and the results obtained allow us to discard in the solids
the accumulation of the reaction products inside the pores of the We have prepared Ti-Beta [31], Sn-Beta [32] and Al-Beta
catalyst.
(Si/Al = 50) [33] considering that Lewis acid catalyzes the DAR
[1]. This effect is known to occur by complexation of the car-
In order to check if the process is diffusion controlled within the bonyl group of the dienophile with the Lewis acid that in-
pores of the zeolite and the reaction is mainly occurring on the creases the electron deficiency of the dienophile, reducing the
external surface, the reaction was carried out with a pure silica energy gap.
nanocrystalline Beta (see Table 1). Table 1 shows differences
between textural characteristics of all studied materials in this The results presented in Figure 2a, b clearly show an important
work. Figure 1 shows, an increase of the reaction rate when increase in activity due to the presence of Brønsted and, espe-
Table 1: Textural characteristic of the studied materials.
Catalyst
Area (m2 g−1)a
Crystallite size (μm)b Metal content (wt %)c External surface (m2 g−1)d
Beta
Nanocrystalline Beta
Ti-Beta
457
595
454
470
518
484
0.5–1
–
24
100
25
0.015-0.02
–
1
1
1.2
1.6
2.8
0.9
Sn-Beta
30
Beta Si/Al = 13
Beta Si/Al = 50
0.1–0.2
0.2
34
50
aArea: Total area of the material per unit of mass. bCrystallite size: Size of the crystalline material (aggregate of a large number of single crystals). It
can vary from a few nanometers to several millimeters. cMetal content: wt percentage of the metal content within the solid structure. dExternal surface:
External area of the material per unit of mass.
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Beilstein J. Org. Chem. 2016, 12, 2181–2188.
Figure 2: Effect of Lewis and Brønsted acid sites in the conversion (a) and selectivity (b) of the DAR.
cially, of Lewis acid sites. Indeed, despite the fact that the crys- the lack of catalytic activity of Beta for the retro-DAR, and elu-
tallite size of Ti- and Sn-Beta zeolites is much larger than cidate whether this is a general effect with zeolites. Due to the
Al-Beta (Table 1), the former give higher conversions.
diffusion limitations with Beta we have selected two extra-large
pore zeolites, SSZ-53 (BET surface area = 377 m2/g) and
Importantly, the catalytic effect on the selectivity of the SSZ-59 (BET surface area = 383 m2/g), with 1D pore system
competing retro-Diels–Alder reaction, which produces an en- and a Si/Al ratio of 49 and 53, respectively. The results given in
hancement of the endo-exo isomer from the endo-endo (see Figure 3 clearly show that the extra-large pore zeolites with
Scheme 1), is much lower for Ti-Beta and even for Al-Beta pore diameters of 8.7 Å and 8.5 Å for SSZ-53 and SSZ-59, re-
zeolites than for MCM-41 [30] (see Figure 2a, b) that owing to spectively, give higher conversions than Beta zeolite, despite
the retro-DA reaction the selectivity of the endo-endo isomer the smaller crystallite size of the last. Interestingly, the retro-
decreases from 85% to 65% as we previously reported. [30]
DAR was neither observed with the two zeolites with extra-
large pores. Similarly to that produced with the silica nanocrys-
Considering the interesting application of beta zeolites as Lewis talline Beta zeolite a concentration effect within the extra-large
acid catalyst for Diels–Alder reactions in different fields, i.e., pore mouth may be responsible of the reaction rate enhance-
the formation of biofuels [34], it is important to get insight into ment observed with SSZ-53 and SSZ-59.
Figure 3: Effect of pore size in the conversion (a) and selectivity (b) of the DAR.
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Therefore, the results seem to confirm that the occurrence of These are aluminosilicogermanates that, as the previously tested
retro-DAR as a competitive reaction not only depends on the Al-zeolites or the Al-MCM-41 material, present Brønsted
presence of Brønsted centers as previously reported for materi- acidity. Interestingly, the pore diameters of ITQ-33, and more
als with lower amounts of Al centers [28], but the structure of so ITQ-37 are close to the pore of the mesoporous MCM-41
the material can play a determinant role. This represents an presented above with 2.0 nm. There is then a unique occasion to
interesting observation since it will imply that, in principle, it compare the catalytic behavior of an amorphous and a crys-
should be possible to change the relative selectivity for DAR talline molecular sieve with practically the same pore diameter
and retro-DAR working with micro or mesoporous catalysts.
(Figure 5a,b).
Thus, Figure 4a,b compares conversion and selectivity to the As observed in Figure 5a,b, the crystalline structure of zeolites
endo-endo isomer with Al-Beta zeolite and the mesoporous ITQ-33 and ITQ-37 do not favor neither the Diels–Alder cyclo-
MCM-41 material previously studied [30] both with very close addition between cyclopentadiene and p-benzoquinone, nor the
Si/Al ratios. It can be observed that both samples give the same retro-Diels–Alder reaction. This result suggests that the reac-
conversion, but different selectivity behavior.
tion takes place on the surface of the material and the pore
structure does not have any influence on the reaction rate,
In the case of the microporous catalyst, Al-Beta zeolite, the neither for the DAR nor for the retro-DAR.
selectivity to the endo-endo isomer remains constant with time,
while with MCM-41 that is formed by larger channels, a contin- To further prove the effect of the structure, the reaction was
uous decrease of the endo-endo with time occurs and the ther- carried out in presence of MCM-41 materials with different
modynamically controlled endo-exo product increases. The Si/Al ratios, and similar pore diameter. The results are collected
retro-Diels–Alder is a consecutive reaction that produces the in Figure 6a,b. As it could be expected no differences were
thermodynamic product and it would occur if there is a certain found in the conversion. Meanwhile, in the case of the selec-
confinement within the pores.
tivity it is possible to observe that increasing the Al content and
lowering the pore size produces an increase in the selectivity of
Thus, it was thought that if retro-DAR occurs in MCM-41 the endo-exo isomer. However, this effect is much more marked
(40 Å), if the pore size is decreased, then this reaction should be when the pore size decreases.
enhanced because of a certain confinement effect through the
reactants. As it can be observed in Figure 4a,b, when the reac- Finally, to conclude the catalytic study of the reaction between
tion was carried out with a mesoporous material of ≈20 Å cyclopentadiene and p-benzoquinone in presence of Beta
instead of 40 Å but with a similar Si/Al ratio, the retro-DAR zeolites, the ability of reuse of Beta Si/Al = 50 was examined.
was enhanced, illustrating a certain confinement effect within As shown in Figure 7a,b the activity of the catalyst decreases in
the pores.
some extension after repeated recycling. As expected for a less
active catalyst, conversion falls partly while the selectivity to
Two extra-large pore 3D zeolites with pore diameters of 1.2 the kinetically controlled endo-endo isomer rises after recy-
(ITQ-33) [35] and 1.9 nm (ITQ-37) [36] have also been tested. cling.
Figure 4: Comparison of conversion (a) and selectivity (b) of the DAR catalysed by Al-Beta zeolite and MCM-41.
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Beilstein J. Org. Chem. 2016, 12, 2181–2188.
Figure 5: Comparison of conversion (a) and selectivity (b) of the DAR catalysed extra-large pore 3D zeolites.
Figure 6: Effect of the Si/Al ratio in the conversion (a) and selectivity (b) of the DAR.
Figure 7: Effect of the reutilization of the catalysts in the conversion (a) and selectivity (b) of the DAR.
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Conclusion
Conversion values for endo-endo and endo-exo products are
In this work the DAR between cyclopentadiene and p-benzo- always referred to conversion from the endo adduct. All com-
quinone has been proved to take place on the catalyst surface pounds were previously described and fully characterized [30].
when the reaction is carried out in presence of microporous ma-
terials, obtaining better results when a smaller crystal size cata- Acknowledgements
lyst is used.
Financial support from the Ministerio de Economía y Competi-
tividad through projects MAT2015-71261-R and CTQ2014-
When Lewis and Brønsted sites are inserted in the material 54987-P, are greatly acknowledged. M. V. Gómez thanks
structure, an improvement of the conversion degree is obtained MINECO for participation in the Ramon y Cajal program. The
as it occurs when MCM-41 and ITQ-2 were used [30]. Howev- authors thank M. Moliner, M. T. Navarro, S. Valencia for pro-
er, a clear change in the selectivity behavior is observed. None viding all the catalytic materials used in this study.
of the used metals showed a retro-DAR enhancing reactivity,
References
1. Yilmaz, Ö.; Kus, N. S.; Tunç, T.; Sahin, E. J. Mol. Struct. 2015, 1098,
even Al, the best hydrogen-bond-donating agent. This result
implies that the competitive retro-DAR takes place not only due
72–75. doi:10.1016/j.molstruc.2015.06.012
to the capability to act as Brønsted sites of metallic centers, but
2. Yamabe, S.; Dai, T.; Minato, T. J. Am. Chem. Soc. 1995, 117,
also due to the structure of MCM-41 and ITQ-2. This effect can
10994–10997. doi:10.1021/ja00149a023
be used in order to obtain a selective product or the other isomer
3. Cativiela, C.; Fraile, J. M.; García, J. I.; Mayoral, J. A.; Pires, E.;
as a result of the chosen catalyst: The more Brønsted sites and
Royo, A. J.; Figueras, F.; de Ménorval, L. C. Tetrahedron 1993, 49,
the more confinement of the reactants, the more retro-DAR will
be observed.
4073–4084. doi:10.1016/S0040-4020(01)89919-1
4. Song, S.; Wu, G.; Dai, W.; Guan, N.; Li, L. J. Mol. Catal. A 2016, 420,
134–141. doi:10.1016/j.molcata.2016.04.023
5. Cativiela, C.; Fraile, J. M.; García, J. I.; Mayoral, J. A.; Figueras, F.;
de Menorval, L. C.; Alonso, P. J. J. Catal. 1992, 137, 394–407.
doi:10.1016/0021-9517(92)90167-G
Experimental
Catalyst preparation
Beta zeolites [pure silica Beta, Beta (Ti), Beta (Sn) and Beta
(Al)] were prepared according to [31-33], using tetraethyl-
ammonium hydroxide as template, tetraethyl orthosilicate
(TEOS) as silica source and Ti(IV) ethoxide, SnCl4·5H2O and
metal Al as sources of heteroatoms. SSZ-53 and SSZ-59 were
synthesized according to the procedures described in the litera-
ture [37-41]. The textural characteristics of the catalysts are
given in Table 1.
6. Figueras, F.; Cativiela, C.; Fraile, J. M.; García, J. I.; Mayoral, J. A.;
de Ménorval, L. C.; Pires, E. Stud. Surf. Sci. Catal. 1994, 83, 391–398.
doi:10.1016/S0167-2991(08)63280-2
7. Eckert, C. A.; Grieger, R. A. J. Am. Chem. Soc. 1970, 92, 7149–7153.
doi:10.1021/ja00727a021
8. Mimoto, A.; Nakano, K.; Ichikawa, Y.; Kotsuki, H. Heterocycles 2010,
80, 799–804. doi:10.3987/COM-09-S(S)85
9. Rideout, D. C.; Breslow, R. J. Am. Chem. Soc. 1980, 102, 7816–7817.
doi:10.1021/ja00546a048
10.Wijnen, J. W.; Engberts, J. B. F. N. J. Org. Chem. 1997, 62,
2039–2044. doi:10.1021/jo9623200
Catalytic tests
11.Moliner, M. Dalton Trans. 2014, 43, 4197–4208.
doi:10.1039/C3DT52293H
In a similar manner as described in [30], after being activated at
250 °C under vacuum (10−2 mm Hg), 250 mg of the corre-
sponding calcined material were introduced into a two necked
bottom flask under N2. Then, 108 mg of p-benzoquinone
(1.0 mmol) and 10.0 mL of CDCl3 were added. The mixture
was stirred at room temperature for a few minutes and 0.2 mL
of freshly distilled cyclopentadiene (3.0 mmol) were added with
a syringe, being this moment considered t = 0 h. The system
was heated at 60 °C and samples were taken every hour, being
directly analyzed by 1H NMR.
12.Climent, M. J.; Corma, A.; Iborra, S. Green Chem. 2014, 16, 516–547.
doi:10.1039/c3gc41492b
13.Anastas, P. T.; Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686–694.
doi:10.1021/ar010065m
14.Brown, S. H. Zeolites in Catalysis. In Handbook of Green Chemistry;
Green Catalysis, Heterogeneous Catalysis; Crabtree, R. H.;
Anastas, P. T., Eds.; Wiley-VCH: Weinheim, Germany, 2009; Vol. 2,
pp 1–50.
15.Corma, A. J. Catal. 2003, 216, 298–312.
doi:10.1016/S0021-9517(02)00132-X
16.Corma, A.; García, H. Chem. Rev. 2003, 103, 4307–4366.
doi:10.1021/cr030680z
Reaction products were isolated by HPLC using mixtures of
H2O/MeOH/MeCN (45:50:5). Identification of these products
was carried out by NMR techniques (1H, 13C, DEPT, COSY,
HETCOR and NOE) being the spectral data fully coincident
with those reported in the literature [42].
17.Koehle, M.; Lobo, R. F. Catal. Sci. Technol. 2016, 6, 3018–3026.
doi:10.1039/C5CY01501D
18.Kang, J.; Rebeck, J., Jr. Nature 1997, 385, 50–52.
doi:10.1038/385050a0
19.Orazov, M.; Davis, M. E. Chem. Sci. 2016, 7, 2264–2274.
doi:10.1039/C5SC03889H
2187

Beilstein J. Org. Chem. 2016, 12, 2181–2188.
20.Chang, C.-C.; Cho, H. J.; Yu, J.; Gorte, R. J.; Gulbinski, J.;
Dauenhauer, P.; Fan, W. Green Chem. 2016, 18, 1368–1376.
doi:10.1039/C5GC02164B
42.Yates, P.; Switlak, K. Can. J. Chem. 1990, 68, 1894–1900.
doi:10.1139/v90-293
21.Zendehdel, M.; Zamani, F.; Khanmohamadi, H.
Microporous Mesoporous Mater. 2016, 225, 552–563.
doi:10.1016/j.micromeso.2016.01.042
License and Terms
22.Onaka, M.; Yamasaki, R. Chem. Lett. 1998, 259–260.
doi:10.1246/cl.1998.259
This is an Open Access article under the terms of the
Creative Commons Attribution License
23.Kugita, T.; Ezawa, M.; Owada, T.; Tomita, Y.; Namba, S.;
Hashimoto, N.; Osaka, M. Microporous Mesoporous Mater. 2001,
44–45, 531–536. doi:10.1016/S1387-1811(01)00231-1
24.Mahmoud, E.; Yu, J.; Gorte, R. J.; Lobo, R. F. ACS Catal. 2015, 5,
6946–6955. doi:10.1021/acscatal.5b01892
(http://creativecommons.org/licenses/by/4.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
25.Green, S. K.; Patet, R. E.; Nikbin, N.; Williams, C. L.; Chang, C.-C.;
Yu, J. Y.; Gorte, R. J.; Caratzoulas, S.; Fan, W.; Vlachos, D. G.;
Dauenhauer, P. J. Appl. Catal., B 2016, 180, 487–496.
doi:10.1016/j.apcatb.2015.06.044
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
(http://www.beilstein-journals.org/bjoc)
26.Corma, A. Catal. Rev.: Sci. Eng. 2004, 46, 369–417.
doi:10.1081/CR-200036732
The definitive version of this article is the electronic one
which can be found at:
27.Pacheco, J. J.; Labinger, J. A.; Sessions, A. L.; Davis, M. E.
ACS Catal. 2015, 5, 5904–5913. doi:10.1021/acscatal.5b01309
28.Pacheco, J. J.; Davis, M. E. Proc. Natl. Acad. Sci. U. S. A. 2014, 111,
8363–8367. doi:10.1073/pnas.1408345111
doi:10.3762/bjoc.12.208
29.Mashayekhi, G.; Ghandi, M.; Farzaneh, F.; Shahidzadeh, M.;
Najafi, H. M. J. Mol. Catal. A: Chem. 2007, 264, 220–226.
doi:10.1016/j.molcata.2006.09.032
30.Gómez, M. V.; Cantín, A.; Corma, A.; de la Hoz, A.
J. Mol. Catal. A: Chem. 2005, 240, 16–21.
doi:10.1016/j.molcata.2005.06.030
31.Blasco, T.; Corma, A.; Esteve, P.; Guil, J. M.; Martinez, A.;
Perdigon-Melon, J. A.; Valencia, S. J. Phys. Chem. B 1998, 102,
75–88. doi:10.1021/jp973288w
32.Corma, A.; Nemeth, L. T.; Renz, M.; Valencia, S. Nature 2001, 412,
423–425. doi:10.1038/35086546
33.Camblor, M. A.; Corma, A.; Valencia, S. J. Mater. Chem. 1998, 8,
2137–2145. doi:10.1039/a804457k
34.Dutta, S.; De, S.; Saha, B.; Alam, M. I. Catal. Sci. Technol. 2012, 2,
2025–2036. doi:10.1039/C2CY20235B
35.Corma, A.; Díaz-Cabañas, M. J.; Jordá, J. L.; Martínez, C.; Moliner, M.
Nature 2006, 443, 842–845. doi:10.1038/nature05238
36.Sun, J.; Bonneau, C.; Cantín, Á.; Corma, A.; Díaz-Cabañas, M. J.;
Moliner, M.; Zhang, D.; Li, M.; Zou, X. Nature 2009, 458, 1154–1157.
doi:10.1038/nature07957
37.Elomari, S.; Zones, S. I. Zeolites and Mesoporous Materials at the
Dawn of the 21st Century. In Proceedings of the 13th International
Zeolite Conference, Vol. 153, Montpellier, France; Elsevier, 2001;
pp 479–486.
38.Chen, C. Y.; Zones, S. I. Preparation of aluminum-, gallium-, and
iron-exchanged borosilicate zeolites and their use as petroleum refining
catalysts. U.S. Patent US 20030133870 A1, July 17, 2003.
39.Burton, A.; Elomari, S.; Chen, C.-Y.; Medrud, R. C.; Chan, I. Y.;
Bull, L. M.; Kibby, C.; Harris, T. V.; Zones, S. I.; Vittoratos, E. S.
Chem. – Eur. J. 2003, 9, 5737–5748. doi:10.1002/chem.200305238
40.Burton, A.; Elomari, S.; Chen, C. Y.; Harris, T. V.; Vittoratos, E. S., Eds.
Recent Advances in the Science and Technology of Zeolites and
Related Materials, Proceedings of the 14th International Zeolite
Conference, Vol. 154A, pp 126–132., Cape Town, South Africa;
Elsevier, 2004.
41.Elomari, S. Zeolite SSZ-53 prepared by using templating agent to be
catalyst. U.S. Patent US 20020104780 A1, Aug 8, 2002.
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