Article pubs.acs.org/cm
Unusual Complete Reduction of Cu2+ Species in Cu-ZSM-5 Zeolites under Vacuum Treatment at High Temperature Manlio Occhiuzzi,*,† Giuseppe Fierro,‡ Giovanni Ferraris,‡ and Giuliano Moretti† †
Dipartimento di Chimica, “SAPIENZA”Università di Roma, Piazzale A. Moro 5, 00185 Roma, Italy Consiglio Nazionale delle Ricerche (CNR)Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), c/o Dipartimento di Chimica, “SAPIENZA”Università di Roma, Piazzale A. Moro 5, 00185 Roma, Italy
‡
S Supporting Information *
ABSTRACT: By electron paramagnetic resonance (EPR), coupled to elemental and thermogravimetric analysis and diffuse reflectance spectroscopy (DRS), we investigated the unusual quasi-complete reduction of Cu2+ species in Cu-ZSM5 zeolites (Si/Al = 80 and 25) at different copper loadings induced by high-temperature vacuum treatments. It has been found that at high temperatures (673−773 K) in vacuum a quasi-complete reduction of Cu2+ unexpectedly occurred in all the “as-prepared Cu-ZSM-5 samples. Evidence is given that such extensive reduction of Cu2+ species is caused by carbonaceous deposits. In order to completely remove any residual carbonaceous species, the “as-prepared” Cu-ZSM-5 materials must be heated in air (or O2) at high temperature (673−773 K) for prolonged time. Differently, some carbon residue, as in particular originated from the organic template used in the zeolite synthesis, can be left within the channels of the ZSM-5 structure. We show also that such a carbon residue, even when escapes the detection accuracy of elemental analysis, can be checked by a careful EPR characterization. KEYWORDS: Cu-ZSM-5 zeolites, reduction of Cu2+ species by carbonaceous deposits, EPR evidence of organic template residue, DRS characterization, EPR characterization
1. INTRODUCTION The transition-metal-ion-containing zeolites represent an important class of materials showing a rather complex chemistry that is particularly evidenced when these solids are used as catalysts. In particular, the Cu-ZSM-5 catalysts have attracted great attention, because they are active and selective for both the NO decomposition and the reduction of NOx by hydrocarbons in the presence of O2,1a−e and, more recently, as a biomimetic inorganic model for methane oxidation.1f Important chemical modification can occur in these solids under practical conditions,2 as aggregation of Cu2+ species with the formation of CuO-like particles within the channels or on the external surface of the zeolite,3 and dealumination.4 Indeed, the nature of the active sites both for NO decomposition and for the selective catalytic reduction of NOx by hydrocarbons in the presence of O2 is still debated.1a−e The chemical and structural properties of the Cu-ZSM-5 catalysts as well as the nature of the copper species are affected by many parameters such as the Si/Al atomic ratio of the starting ZSM-5 matrix, the preparation method, the copper loading, the activation procedure.5 Such an intriguing and complex chemistry, leading to specific local arrangement of the copper species at the extra-framework site of the zeolite, is testified by the fact that the Cu-ZSM-5 catalyst is the only one to be remarkably active for NO decomposition among all the transition-metal-ion-containing zeolites.1a−e,2−10 © 2012 American Chemical Society
A crucial aspect of the chemistry of these materials is related to the reduction of Cu2+ species to Cu+ species, also because this phenomenon has been suggested to play a key role in the NO decomposition mechanism.6−10 We reported that a specific configuration of dimeric Cu+ species at the intersection between the straight and sinusoidal channels of the Cu-ZSM-5 zeolites are able to irreversibly adsorb N2 at low temperature (273−325 K) and could represent the active site for the NO decomposition.6 Also, monomeric Cu+7 or others dimeric (Cu+···Cu+) species,8−10 that must be anyway close to Al−−O−Si framework sites,11,12 have been reported as active sites for this reaction. On the other hand, some authors stated that the Cu2+ species in Cu-ZSM-5 were stable under the reaction conditions.13−15 These different conclusions are not surprising, because a truly coherent description of the NO decomposition catalytic process on Cu-ZSM-5 has proven to be elusive. On the other hand, the reduction of copper species in Cu-ZSM-5 catalysts in vacuum or under a flow of inert gas is a matter of fact. However, the extent of such a reduction and the chemistry behind this process is still lively debated. The extent of reduction is mainly depending on temperature, as well as on Received: December 21, 2011 Revised: April 27, 2012 Published: May 3, 2012 2022
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Cu-ZSM-5 zeolites has been investigated by EPR and DRS spectroscopy, coupled to elemental and thermogravimetric analyses, adding new insights to our previous partial findings.22,23 In particular, the reduction of Cu2+ species was studied for two set of Cu-ZSM-5 samples having different copper loadings. They were prepared from ZSM-5 parent materials characterized by a remarkable difference in the Si/Al ratio (80 and 25) and obtained from two different preparation methods (i.e., with and without the use of the organic template).
the sample history. For instance, an almost complete reduction of the Cu2+ species occurs when “as prepared” Cu-ZSM-5 materials are first treated at high temperature under vacuum or inert gas flow. On the other hand, a much less extent of reduction of the Cu2+ species was observed when the Cu-ZSM-5 samples were initially treated in O2 at high temperature before the treatment under vacuum or inert gas flow at high temperature. Hall and co-workers,8 on the basis of thermobalance experiments, found out that only a small fraction (maximum 20%) of the initial Cu2+ is reduced to Cu+ from switching the gas flow from O2 to pure helium in repeated cycles at 773 K.8c Liu and Robota16 observed by XANES an almost complete reduction of Cu2+ ions to Cu+ after a treatment of as-prepared Cu-ZSM-5 samples in helium at 773 K. It may be worth to recall that, in 1977, Kasai and Bishop17 investigated, by EPR, the reduction of Cu2+ ions in mordenite after activation under vacuum at temperatures of >573 K. These authors evidenced that strongly absorbed water molecules in the coordination sphere of Cu2+ ions played a role either in the reduction process at high temperature, coupled to O2 desorption, and in the reoxidation of Cu+ to Cu2+ at room temperature. When the sample activated under vacuum was exposed at room temperature to degassed water or dry oxygen alone, the Cu2+ signal remained unchanged and increased only in the presence of both O2 and H2O. More recently Zecchina and co-workers have confirmed these results in both “as prepared” Cu-ZSM-5 and Cu-mordenite zeolites concluding that a quasi-complete reduction of Cu2+ to Cu+ ions occurred after a thermal treatment in vacuum in the temperature range of 470−670 K.18a,b If the extent of reduction involves only a partial amount of the total copper, the process has been usually reported in literature as a “self-reduction” process. According to Hall and co-workers,8 a “self-reduction” involves a decrease of the sample weight coupled to O2 desorption with the simultaneous release of four electrons to the solid and consequent reduction of 4 Cu2+ to 4 Cu+ (no metal copper formation was reported by Hall’s group). However, the oxygen source for the “selfreduction” process is questioned. While it has been reported that the “self-reduction” in CuY zeolites occurs only through oxygen atoms of the zeolitic framework,19 in the case of Cu-ZSM-5 materials, Hall8 and Iwamoto9 groups suggested that in the “self-reduction” only extra-framework oxygen species are involved. This idea was also supported by Sachtler5 and Bell20 groups who, in turn, have suggested different mechanisms through which the “self-reduction” process is accomplished, namely, oxygen evolution from [Cu−O−Cu]2+ species or water elimination from [CuOH]+ species, respectively. In the case of a complete reduction of the Cu2+ species in copper-containing zeolites, the situation is much less clear and is still lively debated. It seems to be very unlike that a total reduction of Cu2+ to Cu+ species could be ascribed only to an ‘intrisic’ process, like the “self-reduction”. In this respect, it has been reported that traces of hydrocarbons from the environment may play a role in presence of ionizing radiation, as under XPS measurements.19 However, much more relevant can be the effect of carbonaceous deposits left in the Cu-ZSM-5 zeolites along the preparation. This aspect, to the best of our knowledge, has never been investigated in detail and seems to be missing either in literature dealing with, in general, zeolitebased materials or, specifically, NOx abatement over Cu-ZSM-5 catalysts. The formation of carbonaceous species is welldocumented in relation to acid-catalyzed hydrocarbon reactions.21 In this work, the role of carbonaceous deposits in the
2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. The H-ZSM-5 zeolite with Si/Al = 80 was prepared in three different batches using tetraethylsilicate, tetrapropylammonium hydroxide, and a solution of Al3+ nitrate in ethanol. This mixture was kept under stirring at 333 K for 3 h and then heated, with no stirring, under autogenous pressure in a stainless autoclave at 448 K for 24 h.2,3,6 Then the crystalline product was separated from the mother liquor by centrifugation, washed several times with distilled water and dried for 2 h at 383 K. The dried powder was finally heated in air, with the temperature gradually rising (8.5 K/min) from room temperature up to 823 K and kept at this temperature in air for 5 h, that is the usual standard procedure employed to remove the organic ammine template from the MFI structure.24 At the end of this step, the “as-prepared” H-ZSM-5-80 parent material was obtained. For the sake of comparison, a portion of one of the ZSM-5-80 preparations was not treated in air at high temperature in order to leave the tetrapropylammonium template inside the MFI structure. This solid will be labeled as “as-synthesized” ZSM-5-80. The H-ZSM-5 zeolite with Si/Al = 25 was prepared starting from a commercial product, NH4-ZSM-5 by PQ Corp. (No. CBV5020). The NH4−ZSM-5 was first treated in air at increasing temperature (8.5 K/min from room temperature up to 773 K) and then kept in air at 773 K for 5 h. At the end of this step the “as-prepared” H-ZSM-5-25 parent material was obtained. The Cu-ZSM-5 samples were prepared through a standard ion exchange procedure starting from the “asprepared” H-ZSM-5 parent materials and using diluted cupric acetate, or nitrate, aqueous solutions. In detail, 2.0 g of the “as-prepared” H-ZSM-5 zeolites were added under continuous stirring to 250 mL of the salt solution containing copper in suitable concentration. This ionexchange step was made at 323 K for 2 h. The exchanged samples were washed several time with distilled water, dried for 2 h at 383 K and finally stored in a glass vessel under a controlled relative humidity (ca. 79%). These solids are labeled and hereafter reported like “asprepared” Cu-ZSM-5 samples. In purposely devoted experiments, we found out that if the exchange process is made for longer times this has no effect on the final copper loading. Copper content was determined by atomic adsorption spectroscopy (Varian Model SpectrAA-30). According to a standard nomenclature adopted in literature for these materials, the parent materials and the catalysts will be reported as H-ZSM-5-a and Cu-ZSM-5-a-b where the “a” indicate the Si/Al atom ratio (80 or 25) and “b” the nominal extent of Cu2+ exchange. The extent of Cu2+ exchange was calculated from the Cu/Al atomic ratio [extent of Cu2+ exchange (%) = 2 × (Cu/Al) × 100], assuming that, in any H-ZSM-5 sample, the number of Al3+ ions is equal to the number of H+ ions and that 2 H+ ions can be, in principle, replaced by one Cu2+ ion. Accordingly, an atomic ratio Cu/Al = 0.5 corresponds to 100% of the basic exchange capacity (stoichiometric preparations), while for atomic ratios Cu/Al > 0.5 an overexchanging occurs (overexchanged preparations). Details about the H-ZSM-5 unit cell formula and the calculation of the extent of Cu2+ exchange are reported in the Supporting Information section (Part 1 and 2). 2.2. Catalysts Characterization. 2.2.1. Textural and Elemental Analyses. Textural analysis was performed by N2 adsorption− desorption isotherm at 77 K using a Micromeritics ASAP 2010 analyzer. Before measurements, all the “as-prepared” samples underwent a three-step pretreatment under vacuum: (i) at 423 K for 1 h, (ii) at 523 K for 1 h, and, finally, (iii) at 623 K for 4 h. The “as-synthesized” ZSM-5-80 sample was outgassed at 523 K for 4 h before 2023
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Table 1. Concentration of the Copper Nitrate or Acetate Solutions Used in the Exchange, Copper Content, Cu/Al Ratio, and Copper Exchange Extent for “As-Prepared” H-ZSM-5 and Cu-ZSM-5 Samples sample
Al−−O−Si per MFI unit cell
exchange solutions [Cu2+]a (M)
Cu (wt %)
Cu/Al
extent of Cu2+ exchange (%)
H-ZSM-5-25 Cu-ZSM-5-25-55 Cu-ZSM-5-25-102
3.7 3.7 3.7
0.10 (n) 0.10 (a)
1.02 1.89
0.28 0.51
55 102
H-ZSM-5-80 Cu-ZSM-5-80-84 Cu-ZSM-5-80-225 Cu-ZSM-5-80-540
1.2 1.2 1.2 1.2
0.10 (n) 0.010 (a) 0.10 (a)
0.50 1.35 3.24
0.42 1.13 2.70
84 225 540
In this table, “n” indicates that the sample was prepared using nitrate solution (pH ≅4), and “a” indicates that the sample was prepared using acetate solution (pH ≅5.5).
a
respective spins. Na was then divided by the linear density (g/cm) of the sample in the EPR tube to yield the concentration of paramagnetic species (spins/g). In situ DRS spectra were recorded at room temperature on a Cary Model 5 spectrometer in the wavelength range of 200−2500 nm. A purposely designed cell with a silica window23 permitted heating treatments under vacuum and in O2 (350 Torr) from room temperature up to 790 K.
measurements in order to preserve the integrity of the tetrapropylammonium template within the MFI framework. The Brunauer−Emmett− Teller (BET) specific surface area (St) was calculated using adsorption data in the relative pressure range 0 < P/P0 < 0.1. The micropore volume (Vμ) (i.e., the empty volume in the MFI framework) and the external surface area (Se) (i.e., the area not belonging to micropores in the MFI framework), were determined by t-test25 in the t-range from 5 Å to 15 Å for which the Harkins and Jura reference isotherm equation was used.26 Elemental analyses of carbon, hydrogen, and nitrogen (which should belong to carbonaceous deposits present in the samples) were measured with a Model EA CHNS-O 1110 Carlo Erba analyzer whose accuracy is ±0.01 wt %, ±0.02 wt %, and ±0.02 wt % for C, H and N, respectively. Thermogravimetric analysis (TGA) was performed by a Stanton Redcroft Model STA-781 analyzer under flowing N2 (20 mL/min), increasing the temperature from room temperature up to 993 K at a heating rate of 5 K/min. 2.2.2. Electron Paramagnetic Resonance (EPR) and Diffuse Reflectance Spectroscopy (DRS). The EPR spectra were recorded at room temperature on a Varian E-9 spectrometer (X-band), equipped with a TE102 cavity and an online computer for data processing. Spin-Hamiltonian parameters (g and A values) were obtained from spectra calculated with the program SIM14 A.27 The g-values were computed taking as reference the sharp peak at g = 2.0008 of the E′1 center formed by the UV irradiation of one of the two silica dewars used as sample holders28 (one was irradiated and the other was not irradiated). The EPR spectra were collected on the “as-prepared” catalysts which, after treatment in a vacuum line under different conditions (vide infra), were transferred to the spectrometer in a purposely designed sealed reactor. The treatments in the vacuum line followed this sequence: (a) vacuum at room temperature for 1 h, followed by (b) heating in vacuum at increasing temperatures, from 393 to 773 K for 1 h, followed by (c) treatment at room temperature for 1 h in the presence of H2O (∼20 Torr), followed by (d) treatment at 773 K for 1 h in dry O2 (∼80 Torr). The concentration, as absolute values, of Cu2+ species (i.e., spins/g, ±10%) was calculated from the integrated area of the spectra, taking as standard the Varian strong pitch (5 × 1015 spin/cm). This secondary standard was accurately calibrated by a series of primary standards including Cu(acac)2 (where acac = acetylacetonate) and CuSO4·5H2O in polycrystalline state.29 The specimen, as powder, was placed in the EPR silica tube (i.d. = 3 mm) in a weighted amount to fill the resonant cavity completely in order to have the so-called “full length geometry”. Under these conditions, the numbers of spins/cm (N) are given by the following equation:
⎡ g S (S + 1) ⎤⎛ A ⎞ b b ⎥⎜ a ⎟ Na = Nb⎢ b ⎢⎣ ga Sa(Sa + 1) ⎥⎦⎝ A b ⎠
3. RESULTS AND DISCUSSION For a more comprehensive understanding of the results, a brief reference to the MFI structure30 is given in the Supporting Information (Part 1). 3.1. Chemistry of Base Exchange and Textural Properties. Results concerning the “as-prepared” Cu-ZSM-5 zeolites together with their copper loadings are reported in Table 1. Since the number of H+ ions available for exchange should, in principle, be equal to that of Al3+ ions, it would be expected a higher exchange capacity of H-ZSM-5-25 with respect to the homologous H-ZSM-5-80 matrix. Surprisingly, the data in Table 1 indicate that the opposite occurs. Indeed, regardless the cupric salt (nitrate or acetate) used in the exchange procedure, compared to the H-ZSM-5-25, the H-ZSM-5-80 parent material can be loaded to higher extent of Cu2+ exchange under the same experimental conditions and using a copper acetate solution of the same molarity: the Cu-ZSM-5-80-540 (overexchanged) appears to be loaded to a much higher copper content than the Cu-ZSM-5-25-102 (stoichiometric) sample (see Table 1). These findings can be explained if it is taken into account that single positively charged hydroxy cupric species, Cu(OH)+, are very likely to be involved in the exchanging process. Indeed Cu(OH)+ species have been reported to be the most abundant in copper acetate solutions.3,31,32 Moreover, it has been found that the exchange properties of the H-ZSM-5 zeolites with high Si/Al ratio can also be remarkably influenced by the number of internal silanols, which are defects associated with Si(IV) vacancies in the zeolite framework.30b,c,33 The loss of one framework Si4+ is counterbalanced by four Si−OH groups located around the silicon vacancy. These Si−OH groups are weak Brönsted acid sites with protons (in principle) available for exchange. The number of Si vacancies increases as the Si/Al ratio increases.30b,c,33 It follows that the H-ZSM-5-80 parent material can contain, in its framework, a higher number of silanol defects, compared to the homologous H-ZSM-5-25 matrix, which, in principle, should be free of silanol defects. This can explain why, regardless of the copper salt used (nitrate or acetate), no overexchanged Cu-ZSM-5-25 samples can be prepared while the H-ZSM-5-80 matrix can easily leads to
(1)
where, for the two samples (a and b), Aa and Ab values are the respective integrated areas normalized for the instrumental conditions, ga and gb are the respective average g-values, and Sa and Sb are the 2024
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Table 2. Textural and Elemental Analysis (C, H, and N) Data for H-ZSM-5 Zeolites and Cu-ZSM-5 Samples after Different Thermal Treatments Elemental Analysis (wt %)
a b
sample
treatmenta
St (m2 g−1)
Se (m2 g−1)
Vμ (mL g−1)
C
H
N
H-ZSM-5-25 Cu-ZSM-5-25-55 Cu-ZSM-5-25-102 Cu-ZSM-5-25-102 ZSM-5-80b H-ZSM-5-80 H-ZSM-5-80 Cu-ZSM-5-80-84 Cu-ZSM-5-80-84 Cu-ZSM-5-80-84 H-ZSM-5-80b Cu-ZSM-5-80-225 Cu-ZSM-5-80-225 H-ZSM-5-80b H-ZSM-5-80 Cu-ZSM-5-80-540 Cu-ZSM-5-80-540 Cu-ZSM-5-80-540
“as-prepared” “as-prepared” (n) “as-prepared” (a) air, 823 K “as-synthesized”c “as-prepared” N2, 293−993 K “as-prepared” (n) N2, 293−993 K air, 823 K “as-prepared” “as-prepared” (a) air, 823 K “as-prepared” N2, 293−993 K “as-prepared” (a) N2, 293−993 K air, 823 K
439
20
0.189
441
29
0.180
28 445
28 30
0.000 0.180
438
29
0.178
433 409
23 39
0.174 0.163
435
39
0.176
393
30
0.159
0.11 0.31 0.30 0.00 9.26 0.43 0.00 0.31 0.00 0.03 0.69 0.69 0.03 1.47 0.03 1.33 0.02 0.04
0.64 0.51 0.63 0.63 1.79 0.47 0.36 0.51 0.31 0.38 0.54 0.44 0.34 0.43 0.19 0.59 0.40 0.37
0.00 0.00 0.00 0.00 0.86 0.03 0.00 0.02 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
In this table, “n” indicates that the sample was prepared using nitrate solution, and “a” indicates that the sample was prepared using acetate solution. Three independent preparations of H-ZSM-5-80. cOutgassed at lower temperature (523 K for 4 h).
overexchanged Cu-ZSM-5-80 samples (see Table 1). Moreover, our data clearly show that the cation exchange capacity of the internal silanols is strongly dependent on the solution pH. Indeed, at pH ∼4, i.e., using copper nitrate solutions, it is no longer possible to obtain overexchanged Cu-ZSM-5-80 samples, confirming the weak Brönsted acidity of internal silanols.30c The results of the textural characterization (Table 2) show that the internal and external specific surface area, as well as the micropore volume of both parent H-ZSM-5 zeolites, are only slightly modified by the presence of copper species in all the “as-prepared” Cu-ZSM-5 samples at lower copper content (Cu-ZSM-5-25-102 and Cu-ZSM-5-80-84; see Table 2). By contrast, in the case of the overexchanged catalysts as Cu-ZSM-5-80-225 and especially Cu-ZSM-5-80-540, a decrease of the total specific surface area and of the micropore volume with respect to their H-ZSM-5-80 matrix occurred, this suggesting that CuO-like nanoparticles are entrapped within the zeolite structure.3 It should be noted that, after the high-temperature treatments of overexchanged samples, especially in the presence of water, the CuO nanoclusters tend to move to the external surface of the ZSM-5 zeolite and increase their size.2,3,5,32 3.2. Local Symmetry of Cu2+-Exchanged Species. The EPR spectra of “as-prepared” Cu-ZSM-5-25-102 (Cu = 1.89 wt %) and Cu-ZSM-5-80-225 (Cu = 1.35 wt %), after vacuum treatment at room temperature are reported in Figure 1. The spectra exhibit an axial, broad, and scarcely resolved signal with hyperfine structure in the parallel region. The spectra simulation gave for both samples similar spin-Hamiltonian parameters, which are reported in Table 3. These parameters are typical of isolated Cu 2+ hydrated complexes, i.e., [Cu(H2O)6]2+ and [Cu(H2O)5(OH)]+, in a distorted octahedral symmetry (Table 3: A species, 6-coordinate Cu2+ ions, Cu2+6c).18a,20,32,34−39 These results suggest that, regardless the Si/Al and Cu/Al ratios, the spectra of the “as-prepared” Cu-ZSM-5-25-102 and Cu-ZSM-5-80-225 samples are characterized by almost-identical features.
Figure 1. EPR spectra (experimental and simulated) after vacuum treatment at room temperature: (a) “as-prepared” Cu-ZSM5-25-102 (Cu = 1.89 wt %), (b) “as-prepared” Cu-ZSM-5-80-225 (Cu = 1.35 wt %). The signal assigned to carbon is marked with an asterisk (*).
Table 3. EPR Parameters of Cu2+ Species in Cu-ZSM-5 with Si/Al = 80 and Si/Al = 25 Samples species
g∥
g⊥
A∥
A⊥
Δg∥/Δg⊥
coordination number
A B C D E
2.40 2.32 2.305 2.353 2.268
2.09 2.06 2.06 2.06 2.054
108 156 155 139 169
16 16 16 16 18
4.54 4.54 5.25 5.42 5.18
6c 5c 5c 5c 4c
Figure 2 shows experimental and simulated EPR spectra for the same “as prepared” Cu-ZSM-5 samples of Figure 1 after vacuum treatment at 773 K. Moreover, the subspectra for each assigned spectroscopic species (vide infra) have been also reported. The spectra show a drastic decrease of the signal intensity (due to Cu2+ reduction, vide infra) and become resolved in both parallel and perpendicular regions. This latter feature suggests that a change in Cu2+ coordination occurred when the samples were heated in vacuum. In all Cu-ZSM-5 samples, at lower copper loading (extent of Cu2+ exchange ≤100%), we identified three isolated Cu2+ complexes, labeled 2025
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Figure 2. EPR spectra (experimental and simulated) after vacuum treatment at 773 K of (a) Cu-ZSM-5-25-102 (Cu =1.89 wt %) and (b) Cu-ZSM5-80-225 (Cu = 1.35 wt %). The subspectra of the species used in the simulation are sketched below both the simulated spectra. Magnification of the low field hyperfine structure is presented in the inset. The signal assigned to carbon is marked with an asterisk (*).
zeolites.44 On the other hand, it is worth noting that, in all of our EPR spectra, and regardless of any treatment, no signal at half field due to Cu2+ ion pairs has been detected. This cannot exclude the presence of coupled Cu2+ ions, which, however, if present in our overexchanged Cu-ZSM-5 preparations, are certainly EPR silent. For instance, Cu2+ ion pairs were identified by EPR in CuO45 and in Cu−Y.46 Evidently, the geometry of the Cu2+−O(H)−Cu2+ bonds is unique within the MFI structure giving rise, through the extra-framework oxygen, to strong antiferromagnetic exchange interaction between the coupled Cu2+ spins, which change to EPR-silent species. 3.3. Carbonaceous Residues in H-ZSM-5 and Cu-ZSM-5 Materials and the Reduction of Cu2+ Species. The results of the elemental analyses show that all the “as-prepared” H-ZSM-5 and Cu-ZSM-5 samples have no nitrogen but do contain a significant amount of carbon (see Table 2). Moreover, these data also clearly prove that copper acetate solutions cannot be the source of the carbon contamination, because, for the “as-prepared” Cu-ZSM-5-80 samples obtained via ion exchange in the copper acetate solution, there is no increase of carbon with respect to their “as-prepared” H-ZSM-5-80 parent material. This suggests that the acetate species have been easily eliminated during the final washing of the samples with water. On the other hand, the tetrapropylammonium used as an organic template for the H-ZSM-5-80 preparation, by itself, can be a large reservoir of carbon in the solid. Indeed, in the “as-synthesized” ZSM-5-80 sample, still containing the organic template, carbon amounts to 9.26 wt % and nitrogen to 0.86 wt % (Table 2), data almost perfectly matching the theoretical values of C and N when the ZSM-5 structure is filled with 4 tetrapropylammonium ions per unit cell (u.c.), with 4 being the maximum number allowed by the MFI structure in which 4 channels intersections are present per u.c.30 In order to remove the carbon residue, the “as-synthesized” ZSM-5-80 sample was heated in air at 823 K for 5 h. It should be recalled that such a treatment usually represents the last step in the synthesis of “as-prepared” H-ZSM-5 zeolites.24 Despite heating in air at 823 for 5 h, a significant amount of carbon still remains in the solid (see Table 2), namely, the “as-prepared”
as species B, C, and E (see Figure 2a and parameters in Table 3). In the overexchanged catalysts (the extent of Cu exchange is ≫100%) besides species B and C, one more species (labeled as species D) was detected (see Figure 2b and parameters in Table 3). A further refinement of the assignment of the Cu2+ signals was obtained by investigating how the EPR spectra change when the Cu-ZSM-5 samples, after treatments at 773 K under vacuum or in O2 (vide infra), are contacted with water. In particular, to this purpose, the Cu2+ coordination and the crystal-field axial component have been correlated to two specific spectral features, namely, the Δg∥/Δg⊥ ratio and the A∥ value [ref 40 and references therein]. The 6-coordinated Cu2+ complex (species A) shows the lowest value of both the Δg∥/Δg⊥ ratio and the A∥ parameters, indicating that the other species (B, C, D, and E) have a higher crystal-field axial component and a coordination number lower than 6 (Table 3). Such changes in the symmetry of the Cu2+ ions are not specific for the ZSM-5 zeolite: they also occur in other zeolites.14,15,17,18a,b,20,34−39,41−44 We assign species B, C, and D to isolated copper complexes in square-pyramidal configuration, i.e., Cu2+5c, (possibly differing in the crystal-field symmetry), and the species E to isolated copper complexes in square-planar configuration (Cu2+4c). Accordingly, the signals with g∥ = 2.30− 2.33 were assigned to square-pyramidal 5-coordinated Cu2+ species, while the signals with g∥ = 2.26−2.28 were assigned to square-planar 4-coordinated Cu2+ species.14,15,41,42 Therefore, the EPR investigation confirmed that, as the temperature increases, water ligands are gradually replaced by framework oxygen. This process leads to a decrease in the Cu 2+ coordination, which changes from octahedral into square pyramidal for the overexchanged Cu-ZSM-5 samples and from octahedral into square planar symmetry for the Cu-ZSM-5 samples at lower extent of Cu2+ exchange. It should be noted that when H2O was added to the Cu-ZSM-5-25 and Cu-ZSM-5-80 samples treated under vacuum, species B, C, D, and E changed to species A, namely, the 6-coordinated Cu2+ ion. The above assignments are in agreement with the “Blumberg-Peisach correlation plot approach”, which was used by Larsen and coworkers for the analysis of their EPR spectra of Cu2+-exchanged 2026
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H-ZSM-5-80 parent materials. This finding clearly proves that carbonaceous deposits can be left in the “as-prepared” samples essentially as a consequence of a incomplete burning of the tetrapropylammonium template used in the synthesis in air at high temperature. This is indeed confirmed by the “as-prepared” Cu-ZSM-5-80 samples that, originating from the rich-in-carbon H-ZSM-5-80 parent material, still contain about the same amount of carbon (see Table 2). Also the “as-prepared” H-ZSM-25 and Cu-ZSM-5-25 samples are characterized by the presence of carbon, although in a much lesser amount (see Table 2). In any case, we recall that EPR, because of its high sensitivity, is a very powerful tool for checking traces of carbonaceous deposits that almost always contain paramagnetic carbon species (vide infra).47 Figure 3 reports the EPR spectra of “as-prepared” Cu-ZSM-5 samples at different Si/Al ratio (80 and 25), after heating at
extent of reduction, we have reported, in the Supporting Information (Part 2), the detailed calculation for obtaining the Cu2+ ions and the C atoms per unit cell (u.c.) of the ZSM-5 zeolite starting from their analytical content. In the case of the “as-prepared” Cu-ZSM-5-25-102 sample (1.89 wt % Cu and 0.30 wt % carbon) 1.89 wt % of Cu corresponds to ca. 2 Cu2+ ions per u.c. and 0.30 wt % of C corresponds to ca. 1.5 C atoms per u.c. If, under vacuum and at high temperature, the C atom would reduce the Cu2+ ions to Cu+ forming CO2, each C atom would release 4 electrons, thus reducing 4 Cu2+ ions to Cu+. Therefore, 0.30 wt % of C can release 6 electrons per u.c., which can, in principle, reduce up to 6 Cu2+ ions per u.c. to Cu+. Since, in the u.c., there are only ca. 2 Cu2+ ions, the number of electrons released by C is more than 3 times the number of the Cu2+ ions per u.c. Also, in case CO is formed instead of CO2, the number of electrons released by residual carbon is much larger than that necessary to reduce all Cu2+ ions to Cu+ (3 electron per u.c. from C, versus 2 Cu2+ ions per u.c.). Of course, the situation is far more evident for the “as-prepared” Cu-ZSM-5-80-540 sample, in which the carbon content amounted to 1.33 wt %, corresponding to ∼6.6 C atoms per u.c. (see Table 2). Such a carbon amount, in principle, could reduce up to ca. 26 (or 13) Cu2+ ions per u.c. to Cu+ but in the sample are present only ∼3.2 Cu2+ ions per u.c. The complete reduction of copper by the carbon residue still present in the “as-prepared” solids is supported by the EPR quantitative analysis of the Cu-ZSM-5-25 and Cu-ZSM-5-80 samples treated under vacuum at high temperature (Figure 4), and also confirmed by the DRS analysis (vide infra). After being subjected to a vacuum at room temperature, almost 100% of the initial Cu2+ species can be accounted in the EPR spectra of the Cu-ZSM-5-25-55, Cu-ZSM-5-25-102, and Cu-ZSM-580-84 samples, in which the copper exchanged amounted to ca. 50%, 100%, and 84%, respectively (see Table 1). In other words, the Cu2+ concentration of these three samples, as measured by EPR, agreed with the concentrations obtained by elemental analyses, suggesting that noninteracting Cu2+ species are present after vacuum treatment at room temperature. Upon increasing the temperature up to 473 K under vacuum, the percentage of EPR-detectable Cu2+ species with respect to the “as-prepared” Cu-ZSM-5-25-102 sample decreased (Figure 4a). Only water should be desorbed up to this temperature, which is too low for any sizable reduction of Cu2+ species by the carbonaceous residue, which occurs only at temperatures of >550 K.22,23 Therefore, the observed decrease is probably due to the formation of EPR-silent species, namely Cu2+ complexes with a coordination number of 500 K either under vacuum and in inert gas.22 The EPR analysis of both Cu-ZSM-5-25 and Cu-ZSM-5-80 materials (see Figures 2 and 3) clearly evidenced that some carbonaceous species were still present in the solid, despite the treatment under vacuum at 773 K. As a consequence of this treatment, a drastic decrease of the signal of the Cu2+ species occurred. All these results, overall, represent a strong piece of evidence that the carbonaceous species still present in both sets of the “as-prepared” Cu-ZSM-5 materials are responsible of the observed almost-complete reduction of the Cu2+ species. Simple calculations show that even a very small amount of carbon residue can reduce an amount of copper far greater than its analytical content in the solid. In order to determine the 2027
dx.doi.org/10.1021/cm203796u | Chem. Mater. 2012, 24, 2022−2031
Chemistry of Materials
Article
Figure 4. Percentage of EPR-detectable Cu2+ species with respect to the “as-prepared” sample after different treatments: (a) Cu-ZSM-5-25-55 (Cu = 1.02 wt %) and Cu-ZSM-5-25-102 (Cu = 1.89 wt %) and (b) Cu-ZSM-5-80-84 (Cu = 0.50 wt %), Cu-ZSM-5-80-225 (Cu = 1.35 wt %) and CuZSM-5-80-540 (Cu = 3.24 wt %).
level of the Cu2+ species almost unchanged (see Figure 4a), clearly indicating that, this time, any massive reduction of Cu2+ species did not occur. For the “as-prepared” Cu-ZSM-5-80-84 sample, at lower copper loading, after vacuum treatment at room temperature, ca. 100% of the analytical copper is EPR-active (see Figure 4b) as for the Cu-ZSM-5-25-55 and Cu-ZSM-5-25-102 samples. Upon increasing the temperature under vacuum, the Cu2+ signal intensity of the Cu-ZSM-5-80-84 sample fell gradually from 100% to almost zero at 773 K. After this treatment, a residual carbon was still present in the solid, as clearly evidenced in the EPR spectrum (see Figure 3b). Therefore, as for the Cu-ZSM-5-25-55 and Cu-ZSM-5-25-102 specimens, the gradual disappearing of the Cu2+ signal in the Cu-ZSM-5-80-84 sample by high-temperature (>550 K) vacuum treatments can be mainly ascribed to the reduction of Cu2+ by the carbon residue still present in the solid. Different from the Cu-ZSM-5-80-84 specimen, after the treatment under vacuum at room temperature, the EPR Cu2+ concentration in the “as-prepared” overexchanged Cu-ZSM-580-225 and Cu-ZSM-5-80-540 samples was only ca. 50% and ca. 15% of the total analytical copper, respectively (see Figure 4b). This result suggests that, in these two overexchanged “as-prepared” samples, EPR-silent Cu2+ species are present. The formation of EPR-silent Cu2+ species in Cu-ZSM-5 samples treated under vacuum either at room temperature or at temperatures of
