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NH4-exchanged zeolites: Unexpected catalysts for cyclohexane selective oxidation
Journal Pre-proof NH4-exchanged zeolites: Unexpected catalysts for cyclohexane selective oxidation I. Graça, D. Chadwick PII:
S1387-1811(19)30730-9
DOI:
https://doi.org/10.1016/j.micromeso.2019.109873
Reference:
MICMAT 109873
To appear in:
Microporous and Mesoporous Materials
Received Date: 1 August 2019 Revised Date:
1 November 2019
Accepted Date: 4 November 2019
Please cite this article as: I. Graça, D. Chadwick, NH4-exchanged zeolites: Unexpected catalysts for cyclohexane selective oxidation, Microporous and Mesoporous Materials (2019), doi: https:// doi.org/10.1016/j.micromeso.2019.109873. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
NH4-exchanged zeolites: unexpected catalysts for cyclohexane selective oxidation I. Graça*, D. Chadwick Department of Chemical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK *Corresponding author: [email protected]
Abstract NH4-, H-, Na-, Cs-exchanged ZSM-5 zeolites have been investigated as catalysts for the selective oxidation of cyclohexane under mild conditions using molecular oxygen as oxidant. For comparison, Mn- and Fe-exchanged ZSM-5 zeolites were also studied, as Mn and Fe are well-known oxidation metals. It has been shown that the type of compensating cation in the zeolite framework is of extreme importance for the activity of these catalysts in this reaction. Surprisingly, superior selective oxidation performance was achieved with a commercial NH4ZSM-5 zeolite. The ion-exchanged transition metals (Mn and Fe) were shown to have higher selective oxidation ability compared to the alkali metals, as expected owing to their better redox properties. The rate of cyclohexyl-hydroperoxides transformation into cyclohexanol and cyclohexanone also appears to depend on the ion-exchanged cation, being also much faster over the ammonium and transition metal-exchanged zeolites. Overall, this work has shown for the first time the potential of zeolites ion-exchanged with ammonium to catalyse the selective oxidation of cyclohexane, which in principle offers the possibility of avoiding or reducing the need for more expensive and less environmentally friendly transition metals.
Keywords:
Cyclohexane;
selective
oxidation;
ion-exchanged
zeolites;
KA
oil.
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1. Introduction The activation of C-H bonds through selective oxidation at mild conditions to produce partially oxidised organic compounds for the synthesis of chemicals, plastics and fibres is still one of the most complex and challenging areas in the chemical industry [1-3]. The selective oxidation of cyclohexane to obtain cyclohexanone-cyclohexanol mixtures, known as KA oil, is an industrially important reaction involving the functionalisation of alkane. KA oil is used for the manufacture of nylon polymers [4-6] for which global demand is about 106 ton per year [7]. The current commercial processes for selective oxidation of cyclohexane are performed in the liquid phase under mild conditions (150-160ºC; 10-20 bar of oxygen or air pressure) over homogeneous catalysts (cobalt or manganese salts), yet still present several important drawbacks [4-9]. The processes are operated generally at low conversions (4-6%) to maintain selectivity to KA oil at acceptable levels (70-75%) and in addition they suffer from high waste production. As a consequence, more efficient and sustainable catalytic processes are desirable to meet the continued demand for these oxidation products. The replacement of the homogeneous catalysts by heterogeneous catalysts, which are generally more selective and present lower environmental impact, would be an important step forward in this field. The search for heterogeneous catalysts has led to the development of several promising solid catalytic materials based on transition metals such as Ti, V, Cr, Co, Ni, Zn, Mn, Fe, Mo and Au, which are generally accepted to be the most performant in oxidation reactions [8-13]. However, a few studies in the literature have shown that selective oxidations can also be carried out using alkali or alkali-earth metals supported on zeolites [14-18]. Frei and co-workers [16,17] reported that the oxidation of cyclohexane with molecular oxygen occurs with very high selectivity to cyclohexanone over NaY and BaY zeolites, both under thermal and photocatalytic conditions. Rates of thermal cyclohexane oxidation in both the gas and liquid phases have been identified in the literature to follow the
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trend: CaY > SrY > BaY > NaY [14,15]. This observation was attributed by the authors to the electrostatic field of the cations, as higher electrostatic fields (normally associated to smaller ionic radius) appear to facilitate the O2-hydrocarbon charge transfer. The effect of the type of compensating cation on zeolites was also briefly investigated by Tolman et al. [19] in 1988 when performing the oxidation of mixtures of cyclohexane and cyclododecane over catalysts composed of Fe-phthalocyanine complexes inserted into X zeolites. They observed that the cation had an impact on the selectivity either for the cyclohexane or cyclododecane oxidation. Cyclohexane oxidation was found to be more preponderant over the Fe-complexed NH4exchanged zeolite [19]. Therefore, it seems that the type of compensating cation present on the zeolite framework influences the extent of the cyclohexane oxidation. However, no study has been found comparing the performances of zeolites exchanged with protons, ammonium and alkali metals and also to those of transition metal-supported zeolites, used as typical oxidation catalysts. In particular, the activity of ammonium, without addition of other transition metals, as counter-ion of the zeolite framework has never been properly explored in the literature for this type of reaction. In this work, the solventless selective oxidation of cyclohexane has been investigated over NH4-, H-, Na- and Cs-exchanged (as a small and large alkali cation) ZSM-5 zeolites in the liquid phase at mild conditions using molecular oxygen as oxidant. The use of an intermediate pore zeolite such as ZSM-5 for this study relies on the highest activities being reported in the literature for this type of zeolite in comparison to large or small pore zeolites [13,20]. Mn- and Fe-exchanged ZSM-5 zeolites have been studied to afford a direct comparison with transition metal-exchanged zeolites. It is shown that the commercial NH4ZSM-5 zeolite is catalytically active for the selective oxidation of cyclohexane to KA oil. Indeed, it proved to have a performance exceeding that of transition metal exchanged ZSM-5.
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2. Experimental 2.1 Catalyst preparation Commercial NH4-ZSM-5 (Si/Al=25) zeolite was supplied by Zeolyst with reference CBV 5524G. The protonic form (H+) of the zeolite was obtained through calcination at 500ºC for 6h under air flow. Ion-exchanges of the commercial NH4-ZSM-5 to the Na+, Cs+, Mn2+, Fe3+ zeolite forms were carried out at room temperature for 4h with a solution/zeolite ratio of 4 mL/g, using 2M aqueous solutions of NaNO3 (Sigma-Aldrich, ≥ 99%), CsNO3 (SigmaAldrich, 99%), Mn(NO3)2 (Alfa Aesar, 98%) and Fe(NO3)3 (Sigma-Aldrich, ≥ 98%), respectively. After ion-exchange, the zeolite suspension was filtered under vacuum and the recovered solid was washed with deionised water and dried in an oven overnight at 100°C. The ion-exchange procedure was repeated three times, followed by calcination at 500ºC for 6h under air flow. For comparison, the protonic form of the ZSM-5 zeolite was re-exchanged to the ammonium form at 100°C for 1h with a solution/zeolite ratio of 33 mL/g, using a 0.1M aqueous solutions of NH4NO3 (Sigma-Aldrich, ≥ 99%), for 3 times. Additionally, the commercial ZSM-5 zeolite was subjected to a mild alkaline treatment at 60ºC for 30 min, using a 0.2M NaOH (Sigma-Aldrich, 99%) aqueous solution and a solution/zeolite ratio of 33 mL/g. The alkalitreated ZSM-5 zeolite was then exchanged to the ammonium form using the same conditions as above. The protonic form of the alkali-treated ZSM-5 zeolite was also produced by calcination at 500ºC for 6h under air flow. A summary of the preparation methods of the ZSM-5 zeolite catalysts is given in Table S1, Supporting Information.
2.2 Catalyst characterisation
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XRD patterns were obtained in a PANalytical X’Pert Pro diffractometer, using Cu Kα radiation and operating at 40 kV and 40 mA. The scanning range was set from 10° to 60° (2θ) with a step size of 0.033º and step time of 20s. N2 adsorption measurements were carried out at -196ºC on a Micrometrics TriStar apparatus. Before adsorption, fresh zeolite samples were degassed under vacuum at 90ºC for 1h and then at 350ºC overnight. The total pore volume (Vtotal) was calculated from the adsorbed volume of nitrogen for a relative pressure P/P0 of 0.97, whereas the micropore volume (Vmicro) and the external surface area (Sext) were determined using the t-plot method. The mesopore volume (Vmeso) was given by the difference Vtotal - Vmicro. The Fe content of the commercial NH4-ZSM-5 zeolite was determined by ICP by Intertek. All the other metal contents were determined by ICP-OES by MEDAC Ltd. Analytical data is given in the Supporting Information, Table S2. CHN analysis of the fresh and spent commercial NH4-ZSM-5 zeolite was performed by MEDAC Ltd.
2.3 Catalytic test Cyclohexane oxidation was performed in a 25 mL Büchi AG steel autoclave at 150°C using a PTFE liner, under initial pressure of 10 barg of pure oxygen. Before the reaction, 4 g of cyclohexane (Riedel-de Haën, ≥ 99.5%) and 0.095 g of catalyst were introduced into the reactor. The reactor was sealed, purged with oxygen 3 times and the initial working pressure was set. An oil bath was used to heat up the reactor, which was only introduced after the desired temperature was reached. After immersion in the oil bath, the heating time of the reactor was about 10 min. The reaction was carried out for 4h under continuous stirring (600 rpm) to have a well-mixed condition and avoid external diffusional limitations. After the reaction, the reactor was cooled down using an ice bath, the oxygen was released, and the
5
liquid product-catalyst suspension collected. A blank run was also performed, but no cyclohexane conversion was observed. To analyse the reaction effluent, the catalyst was firstly separated from the mixtures by centrifugation at 5000 rpm for 5 min. All the collected liquid samples were analysed by gas chromatography using a Shimadzu GC-2014 gas chromatograph, with an Agilent CP-Wax 52 CB UltiMetal column and a FID detector. 1,2-dichlorobenzene (Sigma-Aldrich, 99%) was added as standard. Each sample was analysed with and without addition of triphenylphosphine (Sigma-Aldrich, ≥ 95%), which converts the cyclohexyl-hydroperoxides still remaining in the liquid product into the corresponding alcohols [21]. The difference between the cyclohexanol amount after and before triphenylphosphine addition was used to obtain the cyclohexyl-hydroperoxides content. After the product analysis, carbon balances based only on the products cyclohexanol and cyclohexanone were closing at 80-90% due to evaporative losses (mostly cyclohexane) and the difficulty in identifying all the products. Conversion was calculated as the sum of cyclohexanol, cyclohexanone and cyclohexylhydroperoxides yields.
3. Results and Discussion XRD diffraction patterns for the ZSM-5 zeolites are presented in Fig. 1. The characteristic diffraction peaks of the MFI structure can be identified for all the zeolites. No major structural damage is apparent after ion-exchange, as intensities of the diffraction peaks for most of the zeolites remain similar. Only a small decrease in the intensity of the diffraction peaks can be observed for the Cs-ZSM-5 zeolite. According to the literature [22-24], this effect could be attributed to the high X-ray absorption of Cs+, to a partial blockage of the pores or to a decrease in the crystallinity as a result of the incorporation of these large cations. Textural properties of the ZSM-5 zeolites are given in Table 1. No significant changes can be
6
observed due to the ion-exchange procedures, except for the Cs- and Fe-exchanged ZSM-5 zeolites. For the Cs-ZSM-5 zeolite, a decrease in the mesoporous volume and external surface area were noticed, which can be a result of the higher ionic radius of Cs when compared to the other metals. In the case of the Fe-ZSM-5 zeolite, it presents considerably higher microporous and mesoporous volumes, which is most likely caused by a certain degree of erosion by the acidic Fe(NO3)3 aqueous solution. Cyclohexanol, cyclohexanone and cyclohexyl-hydroperoxides were identified as the main products of the selective oxidation of cyclohexane. Fig. 2 shows the conversions to KA oil, expressed as the total yield of cyclohexanol, cyclohexanone and cyclohexyl-hydroperoxides, obtained for the ZSM-5 zeolites after 4h of reaction at 150°C. It can be seen that the activity of the ZSM-5 zeolites ion-exchanged with transition metals (Mn and Fe) is higher than when alkali metals (Na and Cs) are used, even though the former present lower ion-exchange degrees (Table S2, Supporting Information). This is not surprising considering that catalysts based on transition metals possess better redox properties. Among the ZSM-5 zeolites ionexchanged with alkali metals, conversion to KA oil was found to be higher for the Na-ZSM-5 (4.34 %, Fig. 2) than over the Cs-ZSM-5 zeolite (1.56 %, Fig. 2). This observation is in agreement with previous studies [14-18], which claim that cations with lower electrostatic field (higher ionic radius) are less active for the cyclohexane oxidation. In addition, the low conversion found for the Cs-ZSM-5 zeolite could also be partly related to the decrease in the textural parameters observed for this zeolite (Table 1). Interestingly, the acidic form of the commercial ZSM-5 zeolite (prepared by thermal pretreatment) also presents significant oxidation activity. As previously reported [25-28], this activity is associated with the presence of Fe impurities in this zeolite (342 ppm Fe, 0.03 wt%). Indeed, it has been reported in the literature that commercial ZSM-5 zeolites contain mainly lattice Fe (Si-O-Fe, framework) which migrates to extra-framework positions upon
7
thermal treatment, generating extra-framework Fe species, which are found to be much more active than framework Fe for oxidation and N2O decomposition reactions [26,29,30]. These extra-framework Fe species are able to initiate the oxidation reaction mechanism and form the first cyclohexyl-hydroperoxides, even when present in very low amounts. Once the first cyclohexyl-hydroperoxides are formed, the Brønsted acid sites present on the protonic zeolite can promote their subsequent decomposition into the products [15,31]. To confirm the catalytic role of the Fe impurity in cyclohexane oxidation in H-ZSM-5 (Fig. 2) the protonic form of an alkali-treated ZSM-5 zeolite was prepared and tested. Interestingly, only very low residual activity (conversion = 0.02%, Table S3, Supporting Information) was observed for the H-ZSM-5(alkali-treated), even though the alkali-treated ZSM-5 zeolites also appear to contain Fe (0.05%, Table S2, Supporting Information). It has been reported previously that lattice Fe is as easily removed as Al and Si during alkali treatment procedures [32]. It seems, therefore, that the nature of the Fe extra-framework species generated by the alkali-treatment differs from those present in the parent ZSM-5 zeolite, and that they are not active for the reaction. It is reasonable to speculate that the activity of these alkali-treatment generated extra-framework Fe species is inhibited by the presence of the extra-framework Al and Si species also generated during the alkali-treatment, or they are inaccessible to the reactants. Clearly, therefore, the Fe impurity on the H-ZSM-5 produced by direct thermal treatment of the commercial NH4-ZSM-5 zeolite, is responsible for the observed activity of H-ZSM-5 (Fig. 2 and Table S3, Supporting Information). It is quite surprising that the activity of the H-ZSM-5 (3.95 %, Fig. 2) is higher than that of the Cs-ZSM-5 (1.56 %, Fig. 2). However, this could be the result of several factors including the smaller mesoporous volume and external surface area of Cs-ZSM-5 (Table 1) or a less favourable combination of extra-framework Fe and Cs basic sites.
8
The commercial NH4-ZSM-5 zeolite proved to have the best performance for the selective oxidation of cyclohexane (Fig. 2). The activity for this zeolite was even superior to those found for the transition metal-exchanged zeolites. To ascertain whether the presence of the ammonium counter ion is responsible for, or at least correlated with this activity, the NH4ZSM-5(alkali-treated) used to generate the protonic form H-ZSM-5(alkali-treated) zeolite, was also tested in the selective oxidation of cyclohexane. As noted above, the protonic form H-ZSM-5(alkali-treated) gave only a very low activity. However, in contrast, the conversion to KA oil for NH4-ZSM-5(alkali-treated) was 7.09% (Table S3, Supporting Information). While this conversion is lower than the value obtained for the commercial NH4-ZSM-5 zeolite (8.64%), it is clear that ammonium is active itself as compensating cation. However, since the commercial NH4-ZSM-5 zeolite does contain lattice Fe impurity, it is possible that the presence of this lattice Fe impurity enhances the effect of ammonium (and vice versa). An attempt was made to obtain a NH4-ZSM-5 zeolite which has in addition activated Fe species in the following way: the protonic form of ZSM-5 prepared from the commercial NH4-ZSM-5 zeolite by thermal treatment was re-exchanged to give the ammonium form. A conversion to KA oil of 7.39% was found for this re-exchanged NH4-ZSM-5 zeolite (Table S3, Supporting Information). This is much greater than the original H-ZSM-5 (Fig. 2) even though as discussed above this H-ZSM-5 is known to have extra-framework Fe species [26]. Again, this result indicates a significant contribution to the activity by the presence of ammonium as counter ion. However, the activity is slightly lower than found for the commercial NH4-ZSM-5 zeolite, rather than higher as might have been expected. The lower conversion observed for this re-exchanged NH4-ZSM-5 zeolite probably results from leaching of the extra-framework Fe species during ion-exchange to the ammonium form (done in triplicate at 100°C) or perhaps a weak inhibiting interaction between extraframework Fe species and the framework ammonium.
9
Overall, the data confirms that NH4+ can be an active and useful compensating cation when performing the selective oxidation of cyclohexane over zeolites, which is consistent with preliminary results in the literature [19]. This might be related to the fact that NH4+ behaves as a pseudo-alkali metal and presents a much higher electrostatic field even though it has a relatively large ionic radius [33]. The results presented are very interesting, especially considering that NH4-exchanged zeolites have not been much explored in the literature as catalysts. Usually, they are used as intermediates in the preparation of acidic zeolites by thermal decomposition. Therefore, even though they are not stable for high-temperature applications, potentially NH4-exchanged zeolites can be applied under mild operating conditions such as used in the selective oxidation of cyclohexane. Further investigation would be required to delineate the effects of ammonium and iron. For example, the catalytic activity of an ammonium ZSM-5 zeolite synthesised from a purer silica source (no metal impurities) would help to evaluate the separate effect of ammonium as compensating cation. Other possibilities would include the study of a ferrosilicate MFI sieve to analyse the influence of framework Fe alone. The fresh and spent samples of the commercial NH4-ZSM-5 zeolite were analysed by CHN (Table S4, Supporting Information) to ascertain the fate of the ammonium counter ion. The nitrogen contents before and after reaction are similar (0.37 wt%), indicating that no lixiviation of the ammonium ion takes place during cyclohexane oxidation at the conditions used. Nevertheless, a carbon content of 8.36 wt% was found after reaction, indicating that there is deposition of carbonaceous materials on the catalyst during the reaction. After allowing for evaporative losses of cyclohexane, the overall selectivity to KA oil was estimated to be about 90% for the most performant zeolites and close to 100% for zeolites with lower activity. This was calculated considering the carbon balance values (80-90%) and the mass loss by evaporation about 10% (as determined in our previous work [34]). Product
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selectivity considering only the main identified products for selective oxidation of cyclohexane on the ZSM-5 zeolites, after 4h of reaction at 150°C, is given in Table 2. Amount of cyclohexyl-hydroperoxides was observed to be residual for the most active catalysts (NH4-, Mn- and Fe-ZSM-5), whereas cyclohexyl-hydroperoxides selectivities about 20-60% were found for the least performant zeolites (H-, Cs- and Na-ZSM-5). This reveals that the NH4-, Mn- and Fe-ZSM-5 zeolites can promote quickly the transformation of formed cyclohexyl-hydroperoxides into cyclohexanol and cyclohexanone, while this is rather slow over the H-, Cs- and Na-ZSM-5 zeolites. The higher amount of cyclohexyl-hydroperoxides found for the Cs-ZSM-5 zeolite than for the Na-ZSM-5 zeolite is consistent with their intrinsic activity and in agreement with published data on peroxides decomposition over alkali and alkali-earth exchanged Y zeolites [15]. Surprisingly, cyclohexyl-hydroperoxides selectivity for the H-ZSM-5 zeolite is lower than that for the Na-ZSM-5 zeolite, even though activity is higher for the latter (Fig. 2). This observation might indeed confirm the ability of the Brønsted acid sites to convert the cyclohexyl-hydroperoxide into cyclohexanol and cyclohexanone as proposed previously [15,31].
4. Conclusion It has been demonstrated that the type of compensating cation in the ZSM-5 zeolite framework plays a crucial role in the performance of the catalyst for the selective oxidation of cyclohexane at mild conditions. Higher cyclohexane conversions to KA oil are obtained with transition metals than with alkali metals owing to the better redox behaviour of the former. However, commercial NH4-exchanged ZSM-5 zeolite has been shown to exhibit superior performance to transition metals-promoted zeolites for the selective oxidation of cyclohexane. This might be associated with the NH4 pseudo-alkali metal nature and high electrostatic field, possibly also acting in combination with the presence of lattice Fe
11
impurity. The selectivity reveals differences in the rates of transformation of cyclohexylhydroperoxides into cyclohexanol and cyclohexanone. Hence, the potential of NH4exchanged zeolites has been demonstrated for the selective oxidation of cyclohexane. These are cheaper and more environmentally benign catalysts than those based on transition metals.
Acknowledgments This work was performed with financial support from EPSRC(UK) under grant EP/K014749/1. The authors thank Senpei Peng for help with catalyst recovery and analysis.
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https://doi.org/10.1002/anie.201108706 [27] C. Hammond, R. L. Jenkins, N. Dimitratos, J.A. Lopez-Sanchez, M.H. Ab Rahim, M.M. Forde, A. Thetford, D.M. Murphy, H. Hagen, E.E. Stangland, J.M. Moulijn, S.H. Taylor, D.J. Willock, G.J. Hutchings, Catalytic and mechanistic insights of the lowtemperature selective oxidation of methane over Cu-promoted Fe-ZSM-5, Chem. Eur. J. 18 (2012) 15735-15745. https://doi.org/10.1002/chem.201202802 [28] C. Hammond, N. Dimitratos, J.A. Lopez-Sanchez, R.L. Jenkins, G. Whiting, S.A. Kondrat, M.H. Ab Rahim, M.M. Forde, A. Thetford, H. Hagen, E.E. Stangland, J.M. Moulijn, S.H. Taylor, D.J. Willock, G.J. Hutchings, Aqueous phase methane oxidation
16
over Fe-MFI zeolites: promotion through isomorphous framework substitution, ACS Catalysis 3 (2013) 1835-1844. https://doi.org/10.1021/cs400288b [29] J. Pérez‐Ramírez, J. C. Groen, A. Brückner, M. S. Kumar, N. M. Debbagh, U. Bentrup, L. A. Villaescusa, Evolution of isomorphously substituted iron zeolites during activation: comparison of Fe-beta and Fe-ZSM-5, J. Catal. 232 (2005) 232 318-334. https://doi.org/10.1016/j.jcat.2005.03.018 [30] A.H. Oygarden, J. Pérez-Ramírez, Activity of commercial zeolites with iron impurities in direct N2O decomposition, Appl. Catal. B: Environ. 65 (2006) 163-167. https://doi.org/10.1016/j.apcatb.2006.03.002 [31] J. Xu, B.L. Mojet, J.G. van Ommen, L. Lefferts, Propane selective oxidation on alkali earth exchanged zeolite Y: room temperature in situ IR study, Phys. Chem. Chem. Phys. 5 (2003) 4407-4413. https://doi.org/10.1039/B305777C [32] J.C. Groen, L. Maldonado, J.A. Moulijn, J. Pérez-Ramírez, On the role of iron in preparation of mesoporous Fe-MFI zeolites via desilication, Stud. Surf. Sci. Catal. 162 (2006) 267-274. https://doi.org/10.1016/S0167-2991(06)80916-X [33] A. Whiteside, S.S. Xantheas, M. Gutowski, Is Electronegativity a Useful Descriptor for the
Pseudo-Alkali
Metal
NH4?,
Chem.
Eur.
J.
17
(2011)
13197-13205.
https://doi.org/10.1002/chem.201101949 [34] I. Graça, S. Al-Shihri, D. Chadwick, Selective oxidation of cyclohexane: Ce promotion of nanostructured manganese tungstate, Appl. Catal. A: Gen. 568 (2018) 95-104. https://doi.org/10.1016/j.apcata.2018.09.025
17
Table 1. Textural properties for the ZSM-5 zeolites.
a
Catalyst
Vmicro (cm3/g)
Vmeso (cm3/g)
Sext (m2/g)
NH4-ZSM-5a
0.122
0.115
145
H-ZSM-5
0.112
0.124
168
Na-ZSM-5
0.120
0.105
118
Cs-ZSM-5
0.133
0.071
77
Mn-ZSM-5
0.118
0.119
161
Fe-ZSM-5
0.147
0.184
169
Commercial ZSM-5 zeolite.
Table 2. Cyclohexanol, cyclohexanone and cyclohexyl-hydroperoxide selectivities (%) for the ZSM-5 zeolites, at 150°C after 4h.
a
Catalyst
Cyclohexanol
Cyclohexanone
Cyclohexyl-hydroperoxide
NH4-ZSM-5a
47
52
1
H-ZSM-5
47
31
22
Na-ZSM-5
34
22
44
Cs-ZSM-5
25
13
62
Mn-ZSM-5
50
44
6
Fe-ZSM-5
52
46
2
Commercial ZSM-5 zeolite.
18
Figure captions Fig. 1: X-Ray diffractograms for the ZSM-5 zeolites. Fig. 2: Conversion to KA oil for the ZSM-5 zeolites, at 150°C after 4h.
Fig. 1
Fe-ZSM-5
Intensity (a.u.)
Mn-ZSM-5 Cs-ZSM-5 Na-ZSM-5 H-ZSM-5 Commercial NH4-ZSM-5
5
20
35
50
65
80
2θ
Fig. 2
19
Conversion to KA oil (%)
10 9
8.64 7.79
8
6.95
7 6 5 4
3.95
4.34
3 2
1.56
1 0
20
Highlights
•
Cyclohexane selective oxidation on zeolites dependent on compensating cation type.
•
Superior oxidation performance achieved with a commercial NH4-ZSM-5 zeolite.
•
Rate of hydroperoxides’ conversion into KA oil also depends on ion-exchanged cation.
•
Use of NH4 reduces the need for expensive and less sustainable metals.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S1387-1811(19)30730-9
DOI:
https://doi.org/10.1016/j.micromeso.2019.109873
Reference:
MICMAT 109873
To appear in:
Microporous and Mesoporous Materials
Received Date: 1 August 2019 Revised Date:
1 November 2019
Accepted Date: 4 November 2019
Please cite this article as: I. Graça, D. Chadwick, NH4-exchanged zeolites: Unexpected catalysts for cyclohexane selective oxidation, Microporous and Mesoporous Materials (2019), doi: https:// doi.org/10.1016/j.micromeso.2019.109873. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Inc.
NH4-exchanged zeolites: unexpected catalysts for cyclohexane selective oxidation I. Graça*, D. Chadwick Department of Chemical Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK *Corresponding author: [email protected]
Abstract NH4-, H-, Na-, Cs-exchanged ZSM-5 zeolites have been investigated as catalysts for the selective oxidation of cyclohexane under mild conditions using molecular oxygen as oxidant. For comparison, Mn- and Fe-exchanged ZSM-5 zeolites were also studied, as Mn and Fe are well-known oxidation metals. It has been shown that the type of compensating cation in the zeolite framework is of extreme importance for the activity of these catalysts in this reaction. Surprisingly, superior selective oxidation performance was achieved with a commercial NH4ZSM-5 zeolite. The ion-exchanged transition metals (Mn and Fe) were shown to have higher selective oxidation ability compared to the alkali metals, as expected owing to their better redox properties. The rate of cyclohexyl-hydroperoxides transformation into cyclohexanol and cyclohexanone also appears to depend on the ion-exchanged cation, being also much faster over the ammonium and transition metal-exchanged zeolites. Overall, this work has shown for the first time the potential of zeolites ion-exchanged with ammonium to catalyse the selective oxidation of cyclohexane, which in principle offers the possibility of avoiding or reducing the need for more expensive and less environmentally friendly transition metals.
Keywords:
Cyclohexane;
selective
oxidation;
ion-exchanged
zeolites;
KA
oil.
1
1. Introduction The activation of C-H bonds through selective oxidation at mild conditions to produce partially oxidised organic compounds for the synthesis of chemicals, plastics and fibres is still one of the most complex and challenging areas in the chemical industry [1-3]. The selective oxidation of cyclohexane to obtain cyclohexanone-cyclohexanol mixtures, known as KA oil, is an industrially important reaction involving the functionalisation of alkane. KA oil is used for the manufacture of nylon polymers [4-6] for which global demand is about 106 ton per year [7]. The current commercial processes for selective oxidation of cyclohexane are performed in the liquid phase under mild conditions (150-160ºC; 10-20 bar of oxygen or air pressure) over homogeneous catalysts (cobalt or manganese salts), yet still present several important drawbacks [4-9]. The processes are operated generally at low conversions (4-6%) to maintain selectivity to KA oil at acceptable levels (70-75%) and in addition they suffer from high waste production. As a consequence, more efficient and sustainable catalytic processes are desirable to meet the continued demand for these oxidation products. The replacement of the homogeneous catalysts by heterogeneous catalysts, which are generally more selective and present lower environmental impact, would be an important step forward in this field. The search for heterogeneous catalysts has led to the development of several promising solid catalytic materials based on transition metals such as Ti, V, Cr, Co, Ni, Zn, Mn, Fe, Mo and Au, which are generally accepted to be the most performant in oxidation reactions [8-13]. However, a few studies in the literature have shown that selective oxidations can also be carried out using alkali or alkali-earth metals supported on zeolites [14-18]. Frei and co-workers [16,17] reported that the oxidation of cyclohexane with molecular oxygen occurs with very high selectivity to cyclohexanone over NaY and BaY zeolites, both under thermal and photocatalytic conditions. Rates of thermal cyclohexane oxidation in both the gas and liquid phases have been identified in the literature to follow the
2
trend: CaY > SrY > BaY > NaY [14,15]. This observation was attributed by the authors to the electrostatic field of the cations, as higher electrostatic fields (normally associated to smaller ionic radius) appear to facilitate the O2-hydrocarbon charge transfer. The effect of the type of compensating cation on zeolites was also briefly investigated by Tolman et al. [19] in 1988 when performing the oxidation of mixtures of cyclohexane and cyclododecane over catalysts composed of Fe-phthalocyanine complexes inserted into X zeolites. They observed that the cation had an impact on the selectivity either for the cyclohexane or cyclododecane oxidation. Cyclohexane oxidation was found to be more preponderant over the Fe-complexed NH4exchanged zeolite [19]. Therefore, it seems that the type of compensating cation present on the zeolite framework influences the extent of the cyclohexane oxidation. However, no study has been found comparing the performances of zeolites exchanged with protons, ammonium and alkali metals and also to those of transition metal-supported zeolites, used as typical oxidation catalysts. In particular, the activity of ammonium, without addition of other transition metals, as counter-ion of the zeolite framework has never been properly explored in the literature for this type of reaction. In this work, the solventless selective oxidation of cyclohexane has been investigated over NH4-, H-, Na- and Cs-exchanged (as a small and large alkali cation) ZSM-5 zeolites in the liquid phase at mild conditions using molecular oxygen as oxidant. The use of an intermediate pore zeolite such as ZSM-5 for this study relies on the highest activities being reported in the literature for this type of zeolite in comparison to large or small pore zeolites [13,20]. Mn- and Fe-exchanged ZSM-5 zeolites have been studied to afford a direct comparison with transition metal-exchanged zeolites. It is shown that the commercial NH4ZSM-5 zeolite is catalytically active for the selective oxidation of cyclohexane to KA oil. Indeed, it proved to have a performance exceeding that of transition metal exchanged ZSM-5.
3
2. Experimental 2.1 Catalyst preparation Commercial NH4-ZSM-5 (Si/Al=25) zeolite was supplied by Zeolyst with reference CBV 5524G. The protonic form (H+) of the zeolite was obtained through calcination at 500ºC for 6h under air flow. Ion-exchanges of the commercial NH4-ZSM-5 to the Na+, Cs+, Mn2+, Fe3+ zeolite forms were carried out at room temperature for 4h with a solution/zeolite ratio of 4 mL/g, using 2M aqueous solutions of NaNO3 (Sigma-Aldrich, ≥ 99%), CsNO3 (SigmaAldrich, 99%), Mn(NO3)2 (Alfa Aesar, 98%) and Fe(NO3)3 (Sigma-Aldrich, ≥ 98%), respectively. After ion-exchange, the zeolite suspension was filtered under vacuum and the recovered solid was washed with deionised water and dried in an oven overnight at 100°C. The ion-exchange procedure was repeated three times, followed by calcination at 500ºC for 6h under air flow. For comparison, the protonic form of the ZSM-5 zeolite was re-exchanged to the ammonium form at 100°C for 1h with a solution/zeolite ratio of 33 mL/g, using a 0.1M aqueous solutions of NH4NO3 (Sigma-Aldrich, ≥ 99%), for 3 times. Additionally, the commercial ZSM-5 zeolite was subjected to a mild alkaline treatment at 60ºC for 30 min, using a 0.2M NaOH (Sigma-Aldrich, 99%) aqueous solution and a solution/zeolite ratio of 33 mL/g. The alkalitreated ZSM-5 zeolite was then exchanged to the ammonium form using the same conditions as above. The protonic form of the alkali-treated ZSM-5 zeolite was also produced by calcination at 500ºC for 6h under air flow. A summary of the preparation methods of the ZSM-5 zeolite catalysts is given in Table S1, Supporting Information.
2.2 Catalyst characterisation
4
XRD patterns were obtained in a PANalytical X’Pert Pro diffractometer, using Cu Kα radiation and operating at 40 kV and 40 mA. The scanning range was set from 10° to 60° (2θ) with a step size of 0.033º and step time of 20s. N2 adsorption measurements were carried out at -196ºC on a Micrometrics TriStar apparatus. Before adsorption, fresh zeolite samples were degassed under vacuum at 90ºC for 1h and then at 350ºC overnight. The total pore volume (Vtotal) was calculated from the adsorbed volume of nitrogen for a relative pressure P/P0 of 0.97, whereas the micropore volume (Vmicro) and the external surface area (Sext) were determined using the t-plot method. The mesopore volume (Vmeso) was given by the difference Vtotal - Vmicro. The Fe content of the commercial NH4-ZSM-5 zeolite was determined by ICP by Intertek. All the other metal contents were determined by ICP-OES by MEDAC Ltd. Analytical data is given in the Supporting Information, Table S2. CHN analysis of the fresh and spent commercial NH4-ZSM-5 zeolite was performed by MEDAC Ltd.
2.3 Catalytic test Cyclohexane oxidation was performed in a 25 mL Büchi AG steel autoclave at 150°C using a PTFE liner, under initial pressure of 10 barg of pure oxygen. Before the reaction, 4 g of cyclohexane (Riedel-de Haën, ≥ 99.5%) and 0.095 g of catalyst were introduced into the reactor. The reactor was sealed, purged with oxygen 3 times and the initial working pressure was set. An oil bath was used to heat up the reactor, which was only introduced after the desired temperature was reached. After immersion in the oil bath, the heating time of the reactor was about 10 min. The reaction was carried out for 4h under continuous stirring (600 rpm) to have a well-mixed condition and avoid external diffusional limitations. After the reaction, the reactor was cooled down using an ice bath, the oxygen was released, and the
5
liquid product-catalyst suspension collected. A blank run was also performed, but no cyclohexane conversion was observed. To analyse the reaction effluent, the catalyst was firstly separated from the mixtures by centrifugation at 5000 rpm for 5 min. All the collected liquid samples were analysed by gas chromatography using a Shimadzu GC-2014 gas chromatograph, with an Agilent CP-Wax 52 CB UltiMetal column and a FID detector. 1,2-dichlorobenzene (Sigma-Aldrich, 99%) was added as standard. Each sample was analysed with and without addition of triphenylphosphine (Sigma-Aldrich, ≥ 95%), which converts the cyclohexyl-hydroperoxides still remaining in the liquid product into the corresponding alcohols [21]. The difference between the cyclohexanol amount after and before triphenylphosphine addition was used to obtain the cyclohexyl-hydroperoxides content. After the product analysis, carbon balances based only on the products cyclohexanol and cyclohexanone were closing at 80-90% due to evaporative losses (mostly cyclohexane) and the difficulty in identifying all the products. Conversion was calculated as the sum of cyclohexanol, cyclohexanone and cyclohexylhydroperoxides yields.
3. Results and Discussion XRD diffraction patterns for the ZSM-5 zeolites are presented in Fig. 1. The characteristic diffraction peaks of the MFI structure can be identified for all the zeolites. No major structural damage is apparent after ion-exchange, as intensities of the diffraction peaks for most of the zeolites remain similar. Only a small decrease in the intensity of the diffraction peaks can be observed for the Cs-ZSM-5 zeolite. According to the literature [22-24], this effect could be attributed to the high X-ray absorption of Cs+, to a partial blockage of the pores or to a decrease in the crystallinity as a result of the incorporation of these large cations. Textural properties of the ZSM-5 zeolites are given in Table 1. No significant changes can be
6
observed due to the ion-exchange procedures, except for the Cs- and Fe-exchanged ZSM-5 zeolites. For the Cs-ZSM-5 zeolite, a decrease in the mesoporous volume and external surface area were noticed, which can be a result of the higher ionic radius of Cs when compared to the other metals. In the case of the Fe-ZSM-5 zeolite, it presents considerably higher microporous and mesoporous volumes, which is most likely caused by a certain degree of erosion by the acidic Fe(NO3)3 aqueous solution. Cyclohexanol, cyclohexanone and cyclohexyl-hydroperoxides were identified as the main products of the selective oxidation of cyclohexane. Fig. 2 shows the conversions to KA oil, expressed as the total yield of cyclohexanol, cyclohexanone and cyclohexyl-hydroperoxides, obtained for the ZSM-5 zeolites after 4h of reaction at 150°C. It can be seen that the activity of the ZSM-5 zeolites ion-exchanged with transition metals (Mn and Fe) is higher than when alkali metals (Na and Cs) are used, even though the former present lower ion-exchange degrees (Table S2, Supporting Information). This is not surprising considering that catalysts based on transition metals possess better redox properties. Among the ZSM-5 zeolites ionexchanged with alkali metals, conversion to KA oil was found to be higher for the Na-ZSM-5 (4.34 %, Fig. 2) than over the Cs-ZSM-5 zeolite (1.56 %, Fig. 2). This observation is in agreement with previous studies [14-18], which claim that cations with lower electrostatic field (higher ionic radius) are less active for the cyclohexane oxidation. In addition, the low conversion found for the Cs-ZSM-5 zeolite could also be partly related to the decrease in the textural parameters observed for this zeolite (Table 1). Interestingly, the acidic form of the commercial ZSM-5 zeolite (prepared by thermal pretreatment) also presents significant oxidation activity. As previously reported [25-28], this activity is associated with the presence of Fe impurities in this zeolite (342 ppm Fe, 0.03 wt%). Indeed, it has been reported in the literature that commercial ZSM-5 zeolites contain mainly lattice Fe (Si-O-Fe, framework) which migrates to extra-framework positions upon
7
thermal treatment, generating extra-framework Fe species, which are found to be much more active than framework Fe for oxidation and N2O decomposition reactions [26,29,30]. These extra-framework Fe species are able to initiate the oxidation reaction mechanism and form the first cyclohexyl-hydroperoxides, even when present in very low amounts. Once the first cyclohexyl-hydroperoxides are formed, the Brønsted acid sites present on the protonic zeolite can promote their subsequent decomposition into the products [15,31]. To confirm the catalytic role of the Fe impurity in cyclohexane oxidation in H-ZSM-5 (Fig. 2) the protonic form of an alkali-treated ZSM-5 zeolite was prepared and tested. Interestingly, only very low residual activity (conversion = 0.02%, Table S3, Supporting Information) was observed for the H-ZSM-5(alkali-treated), even though the alkali-treated ZSM-5 zeolites also appear to contain Fe (0.05%, Table S2, Supporting Information). It has been reported previously that lattice Fe is as easily removed as Al and Si during alkali treatment procedures [32]. It seems, therefore, that the nature of the Fe extra-framework species generated by the alkali-treatment differs from those present in the parent ZSM-5 zeolite, and that they are not active for the reaction. It is reasonable to speculate that the activity of these alkali-treatment generated extra-framework Fe species is inhibited by the presence of the extra-framework Al and Si species also generated during the alkali-treatment, or they are inaccessible to the reactants. Clearly, therefore, the Fe impurity on the H-ZSM-5 produced by direct thermal treatment of the commercial NH4-ZSM-5 zeolite, is responsible for the observed activity of H-ZSM-5 (Fig. 2 and Table S3, Supporting Information). It is quite surprising that the activity of the H-ZSM-5 (3.95 %, Fig. 2) is higher than that of the Cs-ZSM-5 (1.56 %, Fig. 2). However, this could be the result of several factors including the smaller mesoporous volume and external surface area of Cs-ZSM-5 (Table 1) or a less favourable combination of extra-framework Fe and Cs basic sites.
8
The commercial NH4-ZSM-5 zeolite proved to have the best performance for the selective oxidation of cyclohexane (Fig. 2). The activity for this zeolite was even superior to those found for the transition metal-exchanged zeolites. To ascertain whether the presence of the ammonium counter ion is responsible for, or at least correlated with this activity, the NH4ZSM-5(alkali-treated) used to generate the protonic form H-ZSM-5(alkali-treated) zeolite, was also tested in the selective oxidation of cyclohexane. As noted above, the protonic form H-ZSM-5(alkali-treated) gave only a very low activity. However, in contrast, the conversion to KA oil for NH4-ZSM-5(alkali-treated) was 7.09% (Table S3, Supporting Information). While this conversion is lower than the value obtained for the commercial NH4-ZSM-5 zeolite (8.64%), it is clear that ammonium is active itself as compensating cation. However, since the commercial NH4-ZSM-5 zeolite does contain lattice Fe impurity, it is possible that the presence of this lattice Fe impurity enhances the effect of ammonium (and vice versa). An attempt was made to obtain a NH4-ZSM-5 zeolite which has in addition activated Fe species in the following way: the protonic form of ZSM-5 prepared from the commercial NH4-ZSM-5 zeolite by thermal treatment was re-exchanged to give the ammonium form. A conversion to KA oil of 7.39% was found for this re-exchanged NH4-ZSM-5 zeolite (Table S3, Supporting Information). This is much greater than the original H-ZSM-5 (Fig. 2) even though as discussed above this H-ZSM-5 is known to have extra-framework Fe species [26]. Again, this result indicates a significant contribution to the activity by the presence of ammonium as counter ion. However, the activity is slightly lower than found for the commercial NH4-ZSM-5 zeolite, rather than higher as might have been expected. The lower conversion observed for this re-exchanged NH4-ZSM-5 zeolite probably results from leaching of the extra-framework Fe species during ion-exchange to the ammonium form (done in triplicate at 100°C) or perhaps a weak inhibiting interaction between extraframework Fe species and the framework ammonium.
9
Overall, the data confirms that NH4+ can be an active and useful compensating cation when performing the selective oxidation of cyclohexane over zeolites, which is consistent with preliminary results in the literature [19]. This might be related to the fact that NH4+ behaves as a pseudo-alkali metal and presents a much higher electrostatic field even though it has a relatively large ionic radius [33]. The results presented are very interesting, especially considering that NH4-exchanged zeolites have not been much explored in the literature as catalysts. Usually, they are used as intermediates in the preparation of acidic zeolites by thermal decomposition. Therefore, even though they are not stable for high-temperature applications, potentially NH4-exchanged zeolites can be applied under mild operating conditions such as used in the selective oxidation of cyclohexane. Further investigation would be required to delineate the effects of ammonium and iron. For example, the catalytic activity of an ammonium ZSM-5 zeolite synthesised from a purer silica source (no metal impurities) would help to evaluate the separate effect of ammonium as compensating cation. Other possibilities would include the study of a ferrosilicate MFI sieve to analyse the influence of framework Fe alone. The fresh and spent samples of the commercial NH4-ZSM-5 zeolite were analysed by CHN (Table S4, Supporting Information) to ascertain the fate of the ammonium counter ion. The nitrogen contents before and after reaction are similar (0.37 wt%), indicating that no lixiviation of the ammonium ion takes place during cyclohexane oxidation at the conditions used. Nevertheless, a carbon content of 8.36 wt% was found after reaction, indicating that there is deposition of carbonaceous materials on the catalyst during the reaction. After allowing for evaporative losses of cyclohexane, the overall selectivity to KA oil was estimated to be about 90% for the most performant zeolites and close to 100% for zeolites with lower activity. This was calculated considering the carbon balance values (80-90%) and the mass loss by evaporation about 10% (as determined in our previous work [34]). Product
10
selectivity considering only the main identified products for selective oxidation of cyclohexane on the ZSM-5 zeolites, after 4h of reaction at 150°C, is given in Table 2. Amount of cyclohexyl-hydroperoxides was observed to be residual for the most active catalysts (NH4-, Mn- and Fe-ZSM-5), whereas cyclohexyl-hydroperoxides selectivities about 20-60% were found for the least performant zeolites (H-, Cs- and Na-ZSM-5). This reveals that the NH4-, Mn- and Fe-ZSM-5 zeolites can promote quickly the transformation of formed cyclohexyl-hydroperoxides into cyclohexanol and cyclohexanone, while this is rather slow over the H-, Cs- and Na-ZSM-5 zeolites. The higher amount of cyclohexyl-hydroperoxides found for the Cs-ZSM-5 zeolite than for the Na-ZSM-5 zeolite is consistent with their intrinsic activity and in agreement with published data on peroxides decomposition over alkali and alkali-earth exchanged Y zeolites [15]. Surprisingly, cyclohexyl-hydroperoxides selectivity for the H-ZSM-5 zeolite is lower than that for the Na-ZSM-5 zeolite, even though activity is higher for the latter (Fig. 2). This observation might indeed confirm the ability of the Brønsted acid sites to convert the cyclohexyl-hydroperoxide into cyclohexanol and cyclohexanone as proposed previously [15,31].
4. Conclusion It has been demonstrated that the type of compensating cation in the ZSM-5 zeolite framework plays a crucial role in the performance of the catalyst for the selective oxidation of cyclohexane at mild conditions. Higher cyclohexane conversions to KA oil are obtained with transition metals than with alkali metals owing to the better redox behaviour of the former. However, commercial NH4-exchanged ZSM-5 zeolite has been shown to exhibit superior performance to transition metals-promoted zeolites for the selective oxidation of cyclohexane. This might be associated with the NH4 pseudo-alkali metal nature and high electrostatic field, possibly also acting in combination with the presence of lattice Fe
11
impurity. The selectivity reveals differences in the rates of transformation of cyclohexylhydroperoxides into cyclohexanol and cyclohexanone. Hence, the potential of NH4exchanged zeolites has been demonstrated for the selective oxidation of cyclohexane. These are cheaper and more environmentally benign catalysts than those based on transition metals.
Acknowledgments This work was performed with financial support from EPSRC(UK) under grant EP/K014749/1. The authors thank Senpei Peng for help with catalyst recovery and analysis.
12
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17
Table 1. Textural properties for the ZSM-5 zeolites.
a
Catalyst
Vmicro (cm3/g)
Vmeso (cm3/g)
Sext (m2/g)
NH4-ZSM-5a
0.122
0.115
145
H-ZSM-5
0.112
0.124
168
Na-ZSM-5
0.120
0.105
118
Cs-ZSM-5
0.133
0.071
77
Mn-ZSM-5
0.118
0.119
161
Fe-ZSM-5
0.147
0.184
169
Commercial ZSM-5 zeolite.
Table 2. Cyclohexanol, cyclohexanone and cyclohexyl-hydroperoxide selectivities (%) for the ZSM-5 zeolites, at 150°C after 4h.
a
Catalyst
Cyclohexanol
Cyclohexanone
Cyclohexyl-hydroperoxide
NH4-ZSM-5a
47
52
1
H-ZSM-5
47
31
22
Na-ZSM-5
34
22
44
Cs-ZSM-5
25
13
62
Mn-ZSM-5
50
44
6
Fe-ZSM-5
52
46
2
Commercial ZSM-5 zeolite.
18
Figure captions Fig. 1: X-Ray diffractograms for the ZSM-5 zeolites. Fig. 2: Conversion to KA oil for the ZSM-5 zeolites, at 150°C after 4h.
Fig. 1
Fe-ZSM-5
Intensity (a.u.)
Mn-ZSM-5 Cs-ZSM-5 Na-ZSM-5 H-ZSM-5 Commercial NH4-ZSM-5
5
20
35
50
65
80
2θ
Fig. 2
19
Conversion to KA oil (%)
10 9
8.64 7.79
8
6.95
7 6 5 4
3.95
4.34
3 2
1.56
1 0
20
Highlights
•
Cyclohexane selective oxidation on zeolites dependent on compensating cation type.
•
Superior oxidation performance achieved with a commercial NH4-ZSM-5 zeolite.
•
Rate of hydroperoxides’ conversion into KA oil also depends on ion-exchanged cation.
•
Use of NH4 reduces the need for expensive and less sustainable metals.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: