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SiO2 catalysts prepared by ion exchange for methanol dehydrogenation
i
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 165 (1997) 259-271
Characterization of Cu/SiO2 catalysts prepared by ion exchange for methanol dehydrogenation E.D. Guerreiro, O.F. Gorriz, J.B. Rivarola, L.A. Arrfia* lnstimto de Investigaciones en Tecnolog[a Qulmica (INTEQUI) (UNSL-CONICET), Casilla de Correo 290. 5700 San Luis, Argentina
Received 3 January 1997; received in revised form 4 June 1997; accepted 7 June 1997
Abstract
A series of copper on silica catalysts prepared by using the ion exchange methods with copper loadings between 0.45 and 3 wt% have been studied by means of different techniques such as TPR, EPR, XANES, XRD, XRF, BET and methanol dehydrogenation reaction. The TPR patterns showed two well-defined peaks with maxima between 250-350°C and 600650°C. The first one (peak I) corresponds to two contributions: (i) the one step reduction (Cu 2+ to Cu °) of a low interacted species (Si), and (ii) the partial reduction (Cu2+ to Cu 1+) of a highly dispersed and surface interacted species ($2). The second TPR peak (peak lI) observed at a higher temperature corresponds to the second step reduction (Cu 1+ to Cu°) of the $2 species. In addition, it has been found out that Cu° and not Cu 2+ is the active site for methanol dehydrogenation to methyl formate. ~c, 1997 Elsevier Science B.V. Keywords: Dehydrogenation; Methanol; Methylformate; Copper/silica; Ion exchange
1. I n t r o d u c t i o n Copper on silica catalysts have shown good activities for a variety of reactions such as methanol and ethanol dehydrogenation [1-5], steam reforming of methanol [6,7], ester hydrogenolysis [8], nitrogen oxides transformation [9] and liquid phase hydrolysis of acrylonitrile to acrylamide among others. At least in some of these cases, the copper on silica catalysts prepared by the ion-exchange technique (IE) showed some advantages in that deactivation was significantly reduced as compared to conventional catalysts [8,10]. This effect was attributed to the better dispersion of copper on the support [8,11 ]. With regard to methanol
*Corresponding author. Fax: +54 652 26711. 0926-860X/97/$17.00 (~ 1997 Elsevier Science B.V. All rights reserved. P I I S0926-860X(97)00207-X
dehydrogenation, the Cu/SiO2 prepared by IE has been shown to have much lower deactivation rates than any other type of copper catalysts [1,4,8]. The preparation of these catalysts by IE was first reported by Kobayashi et al. [10[, but a more detailed study has been published by Kohler et al. [ 12]. Several parameters such as copper concentration in the solution, solution pH, type of silica used and washing procedure were found to determine the copper content of the dried catalyst. These catalysts have been also characterized thoroughly by means of various techniques such as TEM, X-ray diffraction analysis, XPS and TPR-TPO [ 13], EPR and UV reflectance [ 14], DTA, XPS, TPR, UV/ VIS/NIR [15,16], IR, AES, magnetic susceptibility [17] and CO and N20 adsorption [18-21] among others.
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The activity of these catalysts has been found to depend mainly on copper loading [6,7] and depending on preparation conditions, it has been suggested that more than one form of copper may exist on the surface [12,13,22,23]. Although the methanol dehydrogenation over Cu/ SiO2 has previously been studied, a detailed study of the activation mechanisms and the identification of the types of copper that undergo reduction during TPR experiments have not appeared. In addition, the oxidation state of the active site in the copper containing catalysts during methanol dehydrogenation to methyl formate is still controversial. Guerrero-Ruiz et al. [5], Tonner et al. [2] and Sodesawa et al. [4] working on different Cu/SiO2 catalysts and Matsuda et al. [24] working with Cu/TSM (fluoro tetrasilicic mica) and Cu/laponite catalysts, suggested that metallic copper is the active site. In contrast, Morikawa et al. [25] studying Cu/TSM prepared by ion exchange and recently Domokos et al. [25] studying Cu/Zr alloys concluded that Cu 2+ ions were responsible for the dehydrogenation of methanol to ME The purpose of the present study has been to investigate how different copper species undergo reduction during the TPR experiments to facilitate the understanding of the activation step, which has shown to be a critical path in the methanol dehydrogenation. TPR, EPR, XANES, XRD, XRF and BET techniques and methanol dehydrogenation reaction were used. A conclusion about the oxidation state of the active site has been achieved,
pH 11.9 for 60 h. The resultant solid was filtered, and then, it was dried with the following steps: (i) 3 h at 50°C and 50 mm Hg, (ii) 2 h at 100°C and 700 mm Hg (iii) 2 h at 100°C and 10 mm Hg. This material was calcinated in air at 450°C for 4 h. This final temperature was reached with a rate of 2~'C rain 1 with an intermediate step of 3 h at 300°C. Thus, seven catalysts were prepared which were identified by 'IE' followed by the nominal Cu wt.% loadings: IE0.5, IE1, IEI.5, IE2, IE2.5, IE3 and IE3.5. A catalyst was prepared by wet-impregnation (WI3) with Cu(NO3)2.3H20 (Fluka p.a) aqueous solution whose concentration was selected in order to achieve 3 Cu wt.% content in the final preparation. The excess of water was removed by evaporation at 70°C in a rotary evaporator. Drying and calcination operation were conducted as above.
2.2. Catalysts characterization 2.2.1. X-ray fluorescence spectroscopy The amount of copper in the catalysts was determined by X-ray fluorescence by using a Philips PW 1400 spectrometer. The calibration curve was made by using standards of concentration measured by atomic absorption spectroscopy (Varian AA-275).
2.2.2. Surface area The BET specific surface areas (SSA, m 2 g - J) were measured by N2 adsorption a t - 196°C (BET method) with a Micromeritics Accusorb 2100 E.
2.2.3. X-ray diffraction 2. Experimental
2.1. Catalyst preparation The reagents used were Aldrich silica (425 m2/g, mean pore radius=28 A, 70-230 mesh, amorphous), Cu(NO3)2.3H20 (Fluka p.a.) and NH4OH (Carlo Erba ASC). The catalysts were prepared by ion exchange as follows. First, an aqueous solution of 0.03 M Cu(NO3)2 was added to an adequate amount of NH4OH. The pH of an aqueous Cu(NH3)42+ complex was adjusted to 11.9 by using NH4OH and by monitoring the pH of the solution with a pH meter (Orion 501). Then, 10 g of silica were added to 20 ml of an aqueous solution of Cu(NH3)4 + complex obtained at
XRD patterns were obtained with a Rigaku diffractometer operated at 35 kV and 30 mA by employing Ni-filtered CuK~t radiation (A--0.51418 nm).
2.2.4. Temperature-programmed reduction (TPR) TPR determinations were carried out in a conventional temperature-programmed reduction system [27]. The reactor was constructed according to the suggestions made by Cvetanovic and Amenomiya [28]. It was placed in close contact with a brass cylinder heated by an electric oven. The temperature was controlled by a temperature-programmed controller.The gas mixture used was 4.5% H2 in N2 at a flow-rate (F) of 30 cm 3 min -I. This mixture was dried in a molecular-sieves trap and purified over an
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259 271
oxitrap. The heating rate (/3) was 10°C min -l. The amount of catalyst used (75--400 mg) depended upon the copper loading so as to keep the ratio t3So/FCo constant [29] (So are moles of copper and Co is hydrogen concentration in moles per cubic centimeter). The analyses were performed by using a conductivity detector. Continuous voltages from the detector cell and reactor thermocouple were converted to digital signals, amplified with a data-acquisition workstation and stored in a PC. 2.2.5. Electron p a r a m a g n e t i c resonance
EPR measurements were made at room temperature on a Bruker spectrometer operated at X-band frequencies. 9.8 GHz Klystron frequency, 100 kHz magnetic field modulation and 10 Gauss/s scan rate were used. The spectrometer was equipped with an on-line computer for data processing. The catalysts were placed in quartz tubes of 4 mm (o.d.), on a bed of quartz-wool, After adequate pretreatment the tubes were sealed with a torch flame. A sealed tube with just the quartz-wool bed was used as blank. 2.2.6. Nitrous oxide chemisorption
In order to determine the copper surface exposed and the dispersion in the reduced samples, the nitrous oxide chemisorption was used. The procedure employed is based on measurement of the hydrogen consumption during temperature-programmed reduction (TPR) after complete bulk oxidation, X, and alter surface oxidation of the same catalyst sample, Y. After complete oxidation, hydrogen consumption provides a measure for the total amount of copper in the catalyst, whereas hydrogen consumption after surface oxidation provides a measure for the number of copper surface atoms. This procedure was, first, proposed by Bond and Namijo [18] and then, it was completed by van der Grift et al. [19]. The last authors also studied the effect of temperature and time for the passivation step and they concluded that copper surface saturation by adsorbed oxygen could be established rapidly by N20 decomposition at temperatures between 90 to 120'C. In our case, surface oxidation (passivation) was performed by dissociative adsorption of nitrous oxide (5% N20/N2) onto the reduced catalyst at 90cC for 20 min. Complete oxidation of the catalyst was carried out with a mixture of 20% O2/N2 at 450~C for 60 min.
261
2.2.7. X A N E S
The XANES experiments were performed by using synchroton radiation at beamline X-18B of the National Synchroton Light Source, Upton, NY. The sample (IE3) was ground with a mortar and pestle and compressed to form a disk which was placed in the cell. The XANES spectra were obtained by measuring the incident and transmitted X-rays in the CuK-edge region. Peaks in the XANES spectrum were found by examining the second derivative of the spectrum. The sample was subjected to a TPR experiment, in which the material was reduced under a stream of 5% H: and 95% N 2 while the temperature was increased from room temperature to 450~'C over 45 min ramp. A XANES spectrum was taken before and after the first TPR peak and eight times during the TPR. Since the collection of a XANES spectrum took approximately 5 min, each spectrum actually represents an approximately 5 0 C range of temperatures. 2.2.8. Catalytic test
Dehydrogenation of methanol was carried out in a conventional fixed bed reactor system. The catalyst (1.0 g) was held in place by a glass-wool sandwich. A methanol/helium mixture at 17/83 molar ratio (total feed rate 30 cm 3 min 1) and 93.32 kPa pressure was used asreactant gas for all the data shown in this work. The reaction temperature was 230C. Analyses of the reaction products were performed with an on-Dine gas chromatograph Hewlett-Packard 5790 A containing a Porapak T column and a flame ionization detector. Complementary analyses were performed with a second gas chromatograph Konik (KNK-3000 HRGC) with a Porapak T column and a thermal conductivity detector. Before activity measurements, the catalysts were activated in two different conditions of time and temperatures {300'C for 60 min and 450 C lot 120 min) with a mixture of 25% HffN>
3. Results With the preparation method used in this work, about 90-95% of the copper complex was taken by the silica from the solution after a contact time of 60 h. This is different from the preparation method employed by Kohler et al. [12] in which a large volume of solution was used. They reported that the
262
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
Table 1 Copper loading and specific surface area of fresh and used catalysts Catalyst
Cu wt%
IE0.5 IE! IE1.5 IE2 IE2.5 IE3 IE3.5
Fresh catalysts
0.45 0.84 1.33 1.71 2.15 2.51 3.01
Used catalysts
SSA (mZ/g)
Pore radius (,~)
SSA (mZ/g)
138 142 132 124 120 114 116
n.d. 76 78 82 80 n.d. 88
n.d. 137 n.d. 128 n.d. 119 114
n.d.: not determined.
ion exchange equilibrium was reached in short time, leaving a high concentration of copper in solution, The experiments of temperature-programmed decomposition (not shown here) were carried out on dried samples to follow the decomposition process of the copper ammonia complex linked to the surface of the silica. The calcination program of the catalysts was defined on this basis, SSA determinations (BET) of fresh and used catalysts are shown in Table 1. It can be observed that the
surface areas of the catalysts are only 25-30% of the surface of the original support while the same surface area was observed for both the fresh and used catalysts. The copper loadings measured by XRF gave in all cases lower concentrations than the nominal ones. The measured values of copper (wt%) percentage are given in Table 1. Fig. 1 shows the XRD diagrams of WI3 and IE3.5 catalysts, both calcinated (fresh) and reduced (used).
**
A
I 0
I
I 20.
I
a-
I 40
~
e"
I
/' ~
0
20
I
I
60
40
,
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60
~
I 80
80
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0
I
0
20
,
I
20
40
,
I
60
,
40
f
60
80
,
I
,
80
20 (degree) Fig. 1. X-ray diffraction patterns of: (a) calcinated (unreduced) WI3 catalyst; (b) used WI3 catalyst; (c) calcinated (unreduced) IE3.5 catalyst and (d) used IE3.5 catalyst. • CuO ;* Cu °.
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
The IE3.5 catalyst, both fresh and used, did not show signal either of metallic copper or of copper oxide (CuO). The IE3.5 catalyst pre-reduced by hydrogen but not used in reaction showed an XRD spectrum identical to that of the used one (Fig. 1(d)). In the case of WI3 catalyst, the typical pattern of CuO was observed in the fresh unreduced catalyst and the Cu ° pattern in the used one. It is commonly accepted that adsorptive decomposition of nitrous oxide is a successful method for determining the free-copper surface area of supported and non-supported copper catalysts. The dissociative absorption proceeds according to the reaction: N20(g) + 2Cu(s) --+ Cu(s)2Oads + N2(g), (1) where Cu(s) denotes a copper surface site. Several analytical techniques have been proposed [30-34] for monitoring the course of the reaction. In this paper we used the procedure described by Bond and Namijo [18] and van der Grift et al. [19] in which copper surface determination is based on measurement of hydrogen consumption after complete oxidation of the catalyst, X, and hydrogenconsumptionafter surface oxidation, Y, of the catalyst. The total amount of reducible copper is measured by means of X (Eq. (2)). The number of copper surface atoms is measured by Y (Eq. (3)), assuming an adsorption stoichiometry of one oxygen atom per two copper surface atoms (Cu~-O-Cus) in order to calculate the number of copper surface atoms from the amount of dissociated nitrous oxide [30,32,35]. Bulk plus surface copper reduction CuO + H2 --+ Cu + H~O hydrogen consumption = X (2)
263
planes gives an average copper surface atom area of 0.0711 nm 2, equivalent to 1.4× 1019 copper atoms per square meter. By assuming a spherical shape of the copper metal particles, S and d .... can be expressed as Eqs. (5) and (6), respectively S = 2 . Y . N a v / ( X " Mcu • 1.4 x 1019) ~- 1 3 5 3 Y / X (m2Cu/gCu), dv.s = 6 / ( S . Pcu) ~ 0.5 • X / Y (nm)
(5) (6)
with Nav=Avogadro's constant, Mcu=relative atomic weight, pcu=copper density--8.92 (g/cm3). Fig. 2 illustrates the typical hydrogen consumption profiles of the completely oxidized and surface-oxidized catalysts (IE1.5 and IE3.5). The second TPR carried out to determine Yafter dissociative adsorption of nitrous oxide showed that hydrogen consumption began at a lower temperature than that of reduction of the completely oxidized particles. This indicated that a new type of oxidized species was being reduced. Dispersion (D), specific copper surface area (S) and average volume-surface diameter of copper particles (dv.~.) obtained for all the catalysts are given in Table 2. The values of dv.~. calculated from the experimental measurements were very small (1.32 to 2.27 nm). Despite the rather good reproducibility of the method, the measurements of D and S did not follow the expected tendency of increase D and S as copper loading decrease. But they showed increasing values of D and S as catalysts loading increased up to IE2. Then, D and S remain almost constant. Complementary experiments were performed with IE1.5 and IE3.5 catalysts. In both cases, the first TPR were stopped at the valley between the two reduction peaks
Surface copper reduction only Cu(s)2Oads q- H2 --~ 2Cu(s)
--H20 hydrogen consumption = Y
(3)
Table 2 Dispersion (D), Specific Copper Surface Area (SI and Average Volume-Surface Diameter of Copper Particles (d, ~.) as a function
Hydrogen consumption (X,Y) can be used to calculate the dispersion (D), the specific copper surface area (S) and the average volume-surface diameter (dv.s.) by means of the following assumptions and equations: D = 2. Y/X (4)
of copper loading
The area per copper surface atom in the (100), (110) and (111) planes are 0.065, 0.092, and 0.0563 nm 2, respectively [36]. An equal abundance of these three
Catalyst
D
S (m 2 Cu/g Cu)
IE 0.5 IEI IEI.5
0.44 0.60 0.60
298 406 406
dw. (nm) 2.27 1.66 1.66
IE2
0.76
514
1.32
IE2.5 IE3
0.74 0.74
501 501
1.35 1.35
IE3.5
0.75
507
1.33
264
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
/.~ IE3.5/
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200
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40
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Fig. 2. Hydrogenconsumptionprofiles of the completelyoxidized (filled symbols) and surface-oxidized(open symbols). ( 0 0 ) IEI.5 and ([S]m) 1E3.5 catalysts.
(referred to as point V) and afterwards, they were passivated with N20. Surface oxidation was not detected for IE1.5 whereas for the IE3.5 some surface oxidation was clearly detected, Temperature-programmed reduction (TPR) has been extensively applied in recent years for characterizing reducible catalysts including metal and metal oxide systems. This technique, therefore, allows a profile or 'finger print' of catalyst reduction to be obtained. This is eminently suitable for studying low loading highly dispersed systems whose characteristics are beyond the limits of detectability by most other direct structural analysis methods (eg. X-ray diffraction). Figs. 3 and 4 depict the TPR spectra obtained over the catalysts with different copper loading. Two peaks are seen with maxima at nearly 250350°C (referred to as peak I) and 600-650°C (peak II). Each peak area can be evaluated by decovolutionintegration of the TPR profile, namely, A1 and An. The low loading catalysts (IE0.5, IE1, IE1.5, IE2) showed AI'~AII while the high loading catalysts (IE2.5, IE3, IE3.5) gave AI clearly bigger than An. As a comparison, TPR of a Cu/SiO2 catalysts prepared by impregnation (WI3) and CuO bulk were carried out. In both cases, there appears only one peak close to peak I (Fig. 4).
Taking into account hydrogen consumption by means of the response calibration performed before and after each TPR, it can be observed that the copper of the impregnated catalyst (WI3) was completely reduced. In contrast, the copper of the IE catalysts was only reduced up to between 85-95%. The IE1.5 and IE3.5 catalysts dried and partially reduced up to different stages were studied by EPR. The partial reduction stages were carried out in the specially modified TPR equipment. The catalysts were treated under the same conditions as TPR expefiments. When the desired level of reduction was reached (determined by monitoring the hydrogen consumed), the flow of the reduction mixture was stopped, purged with nitrogen and the tube was carefully sealed. The results of EPR measurements are given in Fig. 5. The 5a and 5b spectra correspond to dried IE1.5 and IE3.5 catalysts respectively. They showed strong signals in the perpendicular region of the field approximately proportional to copper loading. The IEI.5 catalyst showed values of g//=2.263, A//=178.5 G and g±=2.036 (A± unresolved). The spectra of IE3.5 catalyst could be interpreted as having two types of Cu 2+ ions. The new copper species could be characterized by g//=2.28, A//=187 G and g±=2.06 (A± unresolved). The 5c to
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
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t i m e (rain) Fig. 3. Temperature programmed reduction patterns for IE0.5, IEI, [El.5 and IE2 catalysts.
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/r cu°
,,
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20
40
60
80
100
time (min) Fig. 4. Temperature programmed reduction patterns of IE2.5, IE3, IE3.5 and WI3 catalysts and of bulk CuO.
5h spectra correspond to samples in different stages of reduction. The 5c and 5d spectra performed with the samples partially reduced up to the maximum of peak I (see Figs. 3 and 4) showed a strong decrease of Cu 2+ signal relative to the dried samples. This effect was higher for the IE3.5 than for the IE1.5 catalyst. The
5e and 5f spectra carried out with samples reduced up to point V between the two peaks, showed nearly negligible signals of cupric ions. Finally, the 5g and 5h spectra performed with catalysts reduced to the end of hydrogen consumption, showed no signal of Cu 2+.
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
266 I
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2000
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4000
2000
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4000
gauss (H) Fig. 5. Electron Paramagnetic Resonance spectra of IE1.5 and 1E3.5 catalysts after different pretreatments. (a, b) dried at 100°C for 7 h, (c, d) reduced up to the maximun of peak I heating at 10°C min -1, (e, f) reduced up to point V (between peak I and peak II) heating at 10°C min -I, (g,h) reduced up to the end of hydrogen consumption heating at 10°C min -1.
In addition, a complementary EPR measurement was carried out with an IE3 catalyst completely reduced and treated with N20 for 20 min at 90°C. No signal of Cu 2÷ was observed.
The XANES spectrum of the IE3 catalyst before reduction resembles that of the CuO standard, having peaks at 8977, 8986, 8993 and 8998 eV. The 8977 and 8998 eV peaks have been attributed to ls---~3d and
267
E.D. Guerreiro et al./Applied Catalysis A." General 165 (1997)259-271
1S-+4p transitions, respectively, in other copper (II) c o m p o u n d s [37]. D u r i n g the T P R e x p e r i m e n t the X A N E S spectrum changes appreciably (Fig. 6) with
the original peaks at 8977, 8986 and 8998 eV decreasing and new peaks at 8982 and 9003 eV increasing. The greatest change occurs at ~300c'C temperature
3,0-
~ 2.0,.,
tlllf',\
N
~1.0 0.5
°
_
0,0 8940
8960
8980
I 9020
9000
I 9040
I 9060
L 9080
Energy (eV)
Fig. 6. Normalized XANES spectra of [E3 catalyst taken before, during and alter the first TPR peak.
1 O0
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Cu- Loading (wt%) Fig. 7. Conversion of methanol (filled symbols) and selectivity to methyl formate (open symbols) as a funtion of copper loading. Activation pretreatment with a mixture of 25% H2/N2: (Fq, II) for 1 h at 300'C, (A, A) for 2 h at 450' C.
268
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
range. The peak at 8982 eV is assignable to a l s ~ 4 p transition in Cu(I). Thus, it appears that under the TPR conditions a Cu(II) species is being reduced to a Cu(I) species. Fig. 7 shows the results of methanol conversion (XM) and selectivity to MF (SMF) obtained at 230°C for catalysts with different loading. The activities of the catalysts with lower copper content (IE0.5 to IE2) activated by reduction at 300°C were clearly smaller than those obtained after reduction at 450°C. In contrast, catalysts with higher copper content (IEI.5 to IE3.5) showed very small difference when they were activated by reduction at 300°C or at 450°C. With a higher copper content catalysts it was possible to obtain XM values of about 50-55%. The SMF values (60-65%) were not affected clearly by the activation conditions, In addition, further experiments were carried out on the basis of the information obtained. The reactor used to measure the catalytic activity was adapted to perform in-situ TPR experiments up to different temperatures, and immediately after that to evaluate the catalytic activity. A thermal conductivity detector was used to monitor hydrogen consumption. The methanol conversions obtained with the catalysts IE1 and IE1.5 after reduction up to point V were very small (3-4%) but for IE3.5 it was remarkably higher (X=18%). When the same catalysts (IE1, IE1.5 and IE3.5) were reduced until after peak II and then, evaluated, they showed higher conversions of 22, 29 and 60% respectively,
4. Discussion From data shown in Table 1 it can be seen that SSA of the catalysts are by far lower than the SSA of the support. The preparation method used modifies strongly the porous structure of the support. This could be due to the long-time contact with [Cu(NH3)4] 2+ solution of high pH used in order to favor ionic exchange, Beckler and White [38] showed that XRD was a sensitive tool to define completion of monolayer of different copper complexes on silica. Samples having loadings of the hexanuclear copper complex just 2% in excess of a monolayer showed the presence of peaks characteristic of the polycrystalline metal complex
[38]. With the 3% Cu/SiO2 prepared by wet impregnation (WI3) and used as comparison, the typical CuO pattern was observed when it was unreduced and the Cu ° pattern when it was used. The XRD diagrams of calcinated IE catalysts (unreduced) did not show the typical CuO diffraction lines, thus, indicating that crystals large enough to be observed by XRD were not present, even in the high loading catalysts. The reduced catalysts both fresh and used did not show any signal of metallic copper. This indicates that the copper particle size was very small or amorphous and that sintering process during reaction, if any, was not very important. The values of dv.s. obtained from chemisorption of N20 were very small and are in agreement with the XRD results. The values of D were higher than most of the copper supported catalysts but, in general, they are in agreement with the dispersion mentioned in bibliography for catalysts prepared through similar method [1]. The lower D and S values of the low loading catalysts is not in agreement with the expected trend. It is understandable that catalysts with copper loadings exceeding the ionic exchange capacity of the support (IE2.5, IE3 and 1E3.5) could show, within experimental error, small changes in the D and S values. However, the observed trend of the catalysts with copper loadings below the assumed limit of ionic exchange of the same support is not easy to be explained. It is important to mention that Kohler et al. [12] could not draw meaningful conclusions by using nitrous oxide decomposition to measure copper surface areas of Cu/SiO2 catalysts prepared by ion exchange technique. Moreover, Kenvin and White [39] have reported that highly dispersed 3.8% wt Cu/SiO2 catalyst, prepared with acetylacetonate copper complex, did not react with N20. They speculate that the catalyst does not have ensembles of Cu atoms which are close enough to react with N20 molecules. In this way N20 does not account for isolated Cu. This effect could explain our results considering that part of the copper present in our low loading catalysts could be in similar conditions. In this case the measured dispersions could be underestimated and the values of dv.s. could be overestimated. Trying to get some more evidence about this possibility, a complementary experiment was performed during a catalytic test of IE1.5. In this experiment the catalyst was reduced in H2 at 450°C for 2 h. The methanol feed was then
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
started in the usual way. After 2 h under reaction conditions, the methanol feed was stopped and the temperature of the reactor was lowered to 90°C under nitrogen flow. Then, the oxidation of the copper surface (passivation) with N20 was performed in the same conditions used for dispersion measurements. The reactor was then heated until the reaction temperature under nitrogen flow and the methanol feed restarted. After this treatment, the IE1.5 catalyst showed an initial conversion of approximately 2030% of that measured when the methanol was stopped. After 30rain under these conditions the original activity was fully recovered. This complementary experiment suggests that a fraction of the metallic surface copper is not oxidized by the N20. As Kenvin and White [39] speculate, N20 needs a minimum distance between copper ensembles. If this is so, the lower the catalysts loadings the higher the error, This would explain our unexpected results. In Fig. 7, it can be seen that methanol conversion increases with copper loading. In addition, it is a function of the activation treatment specially for low loading catalysts which require stronger activation conditions to reach their maxima activities. This explains why some low loading catalysts have been reported as low activity catalysts when they have not been properly reduced. The MF selectivities did not show any dependence on the copper loading or the activation conditions, EPR measurements on samples properly conditioned were done in order to follow the evolution of the paramagnetic Cu 2+ species during the reduction of the catalysts. Both IE1.5 and IE3.5 dried catalysts showed strong signals in the perpendicular region of the field approximately proportional to the copper loading. The IE1.5 catalyst showed values of Hamiltonian parameters corresponding to C u 2+ ion in a distorted octahedral crystal field, probably tetragonal [ 14]. The spectra of IE3.5 catalyst could be interpreted as having two types of Cu 2+ ions. The Hamiltonian parameters of this additional species could indicate the presence of Cu 2+ ions coordinated with ammonia and water molecules. Both IE1.5 and IE3.5 catalysts reduced to different levels showed similar shape of the spectra but different intensities. Both of them showed a nearly negligible signal of Cu 2+ when they were previously reduced up to point V.
269
It is clear from the TPR results that the species of copper present in the same support are strongly affected by the loading and preparation methods. The reduction of cupric ions has been studied on different kinds of catalysts by a number of authors. Depending on the support, loading, preparation method and experimental conditions, the TPR presents one, two or three peaks in a wide temperature range (230-800°C). Van der Grift et al. [40], working with catalysts prepared by IE on silica (Aerosil 380 m2/g) did not observe hydrogen consumption at temperatures over 330°C, either with catalyst of 13 wt% or with catalysts containing only l wt % copper. In contrast, Shimokawabe et al. [16] reported that reduction of Cu/SiO2 catalysts prepared by IE to show three peaks at 265-300 (c0, 440 (3), and 650800°C (7). Hydrogen consumption at these temperatures was ascribed to reduction of highly dispersed CuO clusters (c0, isolated Cu(II) ions strongly interacted with de silica surface (c~ y 7), and isolated Cu(II) ions weakly interacted with the support (c~ y 3). When studying Cu/'7-alumina catalysts Baiker et al. [41] identified two types of immobilized cupric species. By using TPR they concluded that the species had very different reducibilities. Two peaks were seen. In both cases it was considered that Cu 2 t was reduced. In agreement with the above assignment, Sermon et al. [42] who researched on the nature of Cu 2+ ions supported upon SiO2, AIO3 and TiO2 using EPR, XPS, XRD, and TPR concluded that divalent copper is stabilized in two different forms, one highly dispersed (with properties detectable by XPS and EPR) and the other poorly dispersed characterized by XRD. Similar results obtained with EPR before and after temperature-programmed reduction of a sample to point between the TPR peaks led the authors to conclude that these peaks do not reflect reduction of Cu 2+ to Cu ~+ and of Cu 1+ to Cu ° but to a reduction of Cu 2+ in two different states to essentially the zerovalent metal. As for zeolite (Cu2+,Na)-X-50 Gentry et al. [43] observed two reduction processes which were assigned to Cu 2+ to Cu -~ reductions in supercage (280~C) and sodalite cage (380°C) positions. As for (Cu2+,Na)-X-20 and (Cu2+,Na)-Y-68, Mahoney et al. [44] and Jacobs et al. [45], respectively, observed two processes assigned to reduction of Cu 2+- to Cu ~ and Cu ~+ to Cu °. The same interpretation was made by Rouco [46] by using other Cu containing catalysts
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prepared by following the incipient wetness procedure on ~/-alumina (Condea Chemie, 190 me/g) and silica (Davison, 340 me/g), At this point, it is important to pay attention to the oxidation state of the active site in these catalysts for the dehydrogenation of methanol to MF, which is still controversial. Guerrero-Ruiz et al. [5], Tonner et al. [2] and Sodesawa et al. [4] working on different Cu/ SiOe catalysts and Matsuda et al. [24] working with Cu/TSM (fluoro tetrasilicic mica) and Cu/laponite catalysts, suggested that metallic copper is the active site. In contrast, Morikawa et al. [25] studying Cu/ TSM prepared by ion exchange and recently Domokos et al. [26] studying Cu/Zr alloys concluded that Cu 2+ ions were responsible for the dehydrogenation of methanol to ME In our case, all the fresh oxidized catalysts studied did not produce MF even after three hours under reaction conditions. All of them showed clear signals of Cu 2+ by EPR at this state. It is clear from these results that Cu 2+ was not the active site for MF formation. Both IE1.5 and IE3.5 showed a nearly negligible signal of Cu e+ by EPR when they were reduced up to point V. At the same level of reduction, the IE3 catalyst studied by XANES showed that the predominant species was Cu +. For low and medium loading catalysts (IE0.5, IE1, IE1.5 and IE2) reduced up to point V, practically no reaction was observed with NeO indicating that the amount of metallic copper was not enough to be detected by this method. The methanol conversions measured with IE1 and IE1.5 catalysts reduced up to point V were very small (3-4%). In addition, the color of the catalysts at this stage was almost white which is characteristic of Cu ÷. From the above comments peak I of the low and medium loading catalysts could be assigned to the reduction of Cu 2+ to Cu ÷. Since the peak areas of these catalysts were very close (AI~An), peak II should be assigned to the reduction of Cu + to Cu °. Taking into account that for IE1 and IEI.5 catalysts reduced up to the end of hydrogen consumption (almost black color) the methanol conversions were 22% and 29%, respectively, it becomes clear that metallic copper was the active site. Moreover, when the IE1.5 catalyst was passivated with N20 during a catalytic test, producing surface oxidation of Cu ° to Cu +, the conversion of methanol was drastically lowered, confirming the previous conclusion,
For high loading catalysts (EI2.5, EI3 and EI3.5) the TPR patterns were similar to those of low and medium loading, but AI was bigger than An. These catalysts reduced up to point V showed an appreciable reaction with N20, and the methanol conversion of IE3.5 measured at that point was 18%. These facts indicate that some amount of metallic copper was present after the first reduction peak. When the IE3.5 catalyst was reduced up to the end of hydrogen consumption, the methanol conversion was 5760%. At this condition, the reaction with NeO was clearly increased. The behavior of high loading catalyts could be explained assuming the presence of other copper species ($2) which reduce from Cu 2+ to Cu ° under peak I. The abundance of Se depends on copper loading and it becomes evident in IE2.5 and over.
5. Conclusions 1. The ion exchange method used in this work affects the texture of the silica by strongly decreasing the SSA. 2. We propose the presence of, at least, two different copper species. The relative abundance of them is a function of the copper loading of the catalysts. Species Sl: It is highly dispersed and strongly interacts with the silica surface. It reduces in two steps (Cu e+ to Cu + and Cu + to Cu °) and in two different temperature ranges. It is abundant in low and medium loading catalysts. Species $2: It weakly interacts with the silica surface. It reduces in only one step (Cu e+ to Cu °) in a temperature range very close to that of Cu 2+ to Cu + reduction of the species $1. Its presence is evident in high loading catalysts. This conclusion is in agreement with the results of Shimokawabe et al. [ 16] for low loading catalysts. For high loading catalyst (IE3.5) we did not observe the/~ peak (440°C). 3. The dissociative chemisorption of N20 is not a proper method to measure the copper surface area on this type of catalysts prepared by ionic exchange and characterized by its high dispersion. 4. The active site for methanol dehydrogenation to methyl formate in this kind of catalysts is Cu ° instead of Cu 2+.
E.D. Guerreiro et al./Applied Catalysis A: General 165 (19971 259-271
Acknowledgements Financial support by C O N I C E T and Universidad Nacional de San Luis are gratefully acknowledged. E.D.G. thanks to CONICET for his fellowship. The authors would like to thank Dr. G. Larsen of University of Nebraska at Lincoln for the XANES measurements, A.M. Gennaro of INTEC for EPR measurements and M.E. Fixman for participating in the TPR determinations.
References [1] T. Sodesawa, React. Kinet. Catal. Lett. 24 (19841 259. [2] S.E Tonner, D.L. Trimm, M.S. Wainwright, Ind. Eng. Chem. Prod. Res. Dev. 23 (19841 384. [3] M. Ai, Appl. Catal. 11 (19841 259. [4] T. Sodesawa, M. Nagacho, A. Onodera, E Nozaki, J. Catal. 102 (19861 460. [5] A. Guerrero-Ruiz, I. Rodriguez-Ramos, J.L,G. Fierro, Appl. Catal. 72 (19911 119. [6] C. Minochi, H. Kobayashi, N. Takesawa, Chem. Lett., (19791 5(17. [7] H. Kobayashi, N. Takesawa, C. Minochi, J. Catal. 69 (19811 487. [8] D.M. Monti, M.S. Wainwright, D.L. Trimm, N.W. Cant, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 397. [9] G. Centi, S. Perathoner, Appl. Catal. A 132 (1995) 179. [1(1] T. Sodesawa, React. Kinet. Catal. Lett. 24 (1984) 259. [11] D.M. Monti, N.W. Cant, D,L. Trimm, M.S. Wainwright, J. Catal. 100 (19861 17. [12] M.A. Kohler, J.C. Lee, D.L. Trimm, N.W. Cant, M.S. Wainwright, Appl. Catal. 31 (1987) 309. [13] M.A. Kohler, H.E. Curry-Hyde, A.E. Hughes, B.A. Sexton. N.W. Cant, J. Catal. 108 (1987)323. [14] H Tominaga, Y. Ono, T. Keii, J. Catal. 40 (1975) 197. [15] M. Shimokawabe, N. Takezawa, H. Kobayashi, Appl. Catal. 2 (19821 379. [16] M. Shimokawabe, N. Takezawa, H. Kobayashi, Bull. Chem. Soc. Jpn. 56 (19831 1337. [17] M. Amara, M. Bettahar, L. Gengembre, D. Olivier, Appl. Catal. 35 (19871 153. [18] G.C. Bond, S.N. Namijo, J. Catal. 118 (19891 507. [19] C.J.G. van der Grift, A.EH. Wielers, B.EJ. Joghi, J. van Beijnum, M. de Boer, M. de Boer, M. Versluijs-Helder, J.W. Geus, J. Catal. 131 (19911 178.
271
[20] N. Takesawa, H. Kobayashi, A. Hirose, M. Shimokawabe, K. Takahashi, Appl. Catal. 4 (1982) 127. [21] N. Takezawa, H. Kobayashi, Y. Kamegai, M. Shimokawabe, Appl. Catal. 3 (1982) 381. [22] M. Shimokawabe, N. Takezawa, H. Kobayashi, Appl. Catal. 2 (19821 379. [23] A. Baiker, D. Monti, A. Wokaum, Appl. Catal. 23 (19861
425.
[24] T, Matsuda, K. Yogo, Ch. Pantawong, E. Kikuchi, Appl. Catal. A 126 (19951 177. [25] Y. Morikawa, Adv. Catal. 39 (19931 303. [26] L. Domokos, T. Katona, A. Molnfir, A. Lovas, Appl. Catal. A 142 (19961 151. [27] D. Monti, A. Baiker, J. Catal. 83 (19831 323. [281 R.J. Cvetanovic, Y. Amenomiya, in: D.D. Eley, H. Pinesand, EB. Weisz (Eds.), Advances in Catalysis, Academic Press, Vol. 17, p. 103, New York, 1967. [29] E Malet, A. Caballero, J. Chem. Soc. Faraday Trans. 1 84(7) (19881 2369. [30] J.J.F. Scholten, J.A. Konvalinka. Trans. Faraday Soc. 65 (19691 2465. [31] B, DvorS.k, J. Pasek, J. Catal. 18 (19701 1(/8. [32] J.W. Evans, M.S. Wainwright, A.J. Bridgewater, D.J. Young, Appl. Catal. 7 (19831 75. [33] G.C. Chinchen, C.M. Hay, H.D. Vandervell, K.C. Waugh, J. Catal. 103 (19871 79. [34] E, Giamello, B. Fubini, P. Lauro, A. Bossi, J. Catal. 87 (19841 443. [35] M.J. Luys, P.H. van Oeffelt, W.G.J. Brouwer, A.P. Pijpers. J.J.E Scholten, Appl. Catal. 46 (19891 161. [36] G.L. Kington. J.M. Holmes, Trans. Faraday Soc. 49 (19531 417. [37] S.K. Pandey, A.R. Chetal, ER. Sarode, Phys. B 172 11991) 324. [38] R.K. Beckler, G. White Mark, J. Catal. 102 (19861 252. [39] J.C. Kenvin, M.G. White, J. Catal. 13(/(19911447. [40] C.J. van der Grift, A. Mulder, J.W. Geus, Appl. Catal. 60 (19901 181. [41] A. Baiker, D. Monti, A. Wokaun, Appl. Catal. 23 (19861 425. [42] P.A. Sermon, K. Rollins, EN. Reyes, S.A. Lawrence, M.A. Martin Luengo, M.J. Davies, J. Chem. Soc., Faraday Trans. 1 83 (19871 1347. [43] S.J. Gentry, N.W. Hurst, A. Jones, J. Chem. Soc.. Faraday Trans. 1 75 (19791 1688. [44] E Mahoney, R. Rudham. J.V. Summers, J. Chem. Soc.. Faraday Trans. 1 75 (19791 314. [45] EA. Jacobs, J.E Linart, H. Nijs, J.B. Uytterhoeven. J. Chem. Sot., Faraday Trans. 1 73 (19771 1745. [46] A.J. Rouco, Appl. Catal. A 117 (19941 139.
A PT PA LE IY DSS CA L I A: GENERAL
ELSEVIER
Applied Catalysis A: General 165 (1997) 259-271
Characterization of Cu/SiO2 catalysts prepared by ion exchange for methanol dehydrogenation E.D. Guerreiro, O.F. Gorriz, J.B. Rivarola, L.A. Arrfia* lnstimto de Investigaciones en Tecnolog[a Qulmica (INTEQUI) (UNSL-CONICET), Casilla de Correo 290. 5700 San Luis, Argentina
Received 3 January 1997; received in revised form 4 June 1997; accepted 7 June 1997
Abstract
A series of copper on silica catalysts prepared by using the ion exchange methods with copper loadings between 0.45 and 3 wt% have been studied by means of different techniques such as TPR, EPR, XANES, XRD, XRF, BET and methanol dehydrogenation reaction. The TPR patterns showed two well-defined peaks with maxima between 250-350°C and 600650°C. The first one (peak I) corresponds to two contributions: (i) the one step reduction (Cu 2+ to Cu °) of a low interacted species (Si), and (ii) the partial reduction (Cu2+ to Cu 1+) of a highly dispersed and surface interacted species ($2). The second TPR peak (peak lI) observed at a higher temperature corresponds to the second step reduction (Cu 1+ to Cu°) of the $2 species. In addition, it has been found out that Cu° and not Cu 2+ is the active site for methanol dehydrogenation to methyl formate. ~c, 1997 Elsevier Science B.V. Keywords: Dehydrogenation; Methanol; Methylformate; Copper/silica; Ion exchange
1. I n t r o d u c t i o n Copper on silica catalysts have shown good activities for a variety of reactions such as methanol and ethanol dehydrogenation [1-5], steam reforming of methanol [6,7], ester hydrogenolysis [8], nitrogen oxides transformation [9] and liquid phase hydrolysis of acrylonitrile to acrylamide among others. At least in some of these cases, the copper on silica catalysts prepared by the ion-exchange technique (IE) showed some advantages in that deactivation was significantly reduced as compared to conventional catalysts [8,10]. This effect was attributed to the better dispersion of copper on the support [8,11 ]. With regard to methanol
*Corresponding author. Fax: +54 652 26711. 0926-860X/97/$17.00 (~ 1997 Elsevier Science B.V. All rights reserved. P I I S0926-860X(97)00207-X
dehydrogenation, the Cu/SiO2 prepared by IE has been shown to have much lower deactivation rates than any other type of copper catalysts [1,4,8]. The preparation of these catalysts by IE was first reported by Kobayashi et al. [10[, but a more detailed study has been published by Kohler et al. [ 12]. Several parameters such as copper concentration in the solution, solution pH, type of silica used and washing procedure were found to determine the copper content of the dried catalyst. These catalysts have been also characterized thoroughly by means of various techniques such as TEM, X-ray diffraction analysis, XPS and TPR-TPO [ 13], EPR and UV reflectance [ 14], DTA, XPS, TPR, UV/ VIS/NIR [15,16], IR, AES, magnetic susceptibility [17] and CO and N20 adsorption [18-21] among others.
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The activity of these catalysts has been found to depend mainly on copper loading [6,7] and depending on preparation conditions, it has been suggested that more than one form of copper may exist on the surface [12,13,22,23]. Although the methanol dehydrogenation over Cu/ SiO2 has previously been studied, a detailed study of the activation mechanisms and the identification of the types of copper that undergo reduction during TPR experiments have not appeared. In addition, the oxidation state of the active site in the copper containing catalysts during methanol dehydrogenation to methyl formate is still controversial. Guerrero-Ruiz et al. [5], Tonner et al. [2] and Sodesawa et al. [4] working on different Cu/SiO2 catalysts and Matsuda et al. [24] working with Cu/TSM (fluoro tetrasilicic mica) and Cu/laponite catalysts, suggested that metallic copper is the active site. In contrast, Morikawa et al. [25] studying Cu/TSM prepared by ion exchange and recently Domokos et al. [25] studying Cu/Zr alloys concluded that Cu 2+ ions were responsible for the dehydrogenation of methanol to ME The purpose of the present study has been to investigate how different copper species undergo reduction during the TPR experiments to facilitate the understanding of the activation step, which has shown to be a critical path in the methanol dehydrogenation. TPR, EPR, XANES, XRD, XRF and BET techniques and methanol dehydrogenation reaction were used. A conclusion about the oxidation state of the active site has been achieved,
pH 11.9 for 60 h. The resultant solid was filtered, and then, it was dried with the following steps: (i) 3 h at 50°C and 50 mm Hg, (ii) 2 h at 100°C and 700 mm Hg (iii) 2 h at 100°C and 10 mm Hg. This material was calcinated in air at 450°C for 4 h. This final temperature was reached with a rate of 2~'C rain 1 with an intermediate step of 3 h at 300°C. Thus, seven catalysts were prepared which were identified by 'IE' followed by the nominal Cu wt.% loadings: IE0.5, IE1, IEI.5, IE2, IE2.5, IE3 and IE3.5. A catalyst was prepared by wet-impregnation (WI3) with Cu(NO3)2.3H20 (Fluka p.a) aqueous solution whose concentration was selected in order to achieve 3 Cu wt.% content in the final preparation. The excess of water was removed by evaporation at 70°C in a rotary evaporator. Drying and calcination operation were conducted as above.
2.2. Catalysts characterization 2.2.1. X-ray fluorescence spectroscopy The amount of copper in the catalysts was determined by X-ray fluorescence by using a Philips PW 1400 spectrometer. The calibration curve was made by using standards of concentration measured by atomic absorption spectroscopy (Varian AA-275).
2.2.2. Surface area The BET specific surface areas (SSA, m 2 g - J) were measured by N2 adsorption a t - 196°C (BET method) with a Micromeritics Accusorb 2100 E.
2.2.3. X-ray diffraction 2. Experimental
2.1. Catalyst preparation The reagents used were Aldrich silica (425 m2/g, mean pore radius=28 A, 70-230 mesh, amorphous), Cu(NO3)2.3H20 (Fluka p.a.) and NH4OH (Carlo Erba ASC). The catalysts were prepared by ion exchange as follows. First, an aqueous solution of 0.03 M Cu(NO3)2 was added to an adequate amount of NH4OH. The pH of an aqueous Cu(NH3)42+ complex was adjusted to 11.9 by using NH4OH and by monitoring the pH of the solution with a pH meter (Orion 501). Then, 10 g of silica were added to 20 ml of an aqueous solution of Cu(NH3)4 + complex obtained at
XRD patterns were obtained with a Rigaku diffractometer operated at 35 kV and 30 mA by employing Ni-filtered CuK~t radiation (A--0.51418 nm).
2.2.4. Temperature-programmed reduction (TPR) TPR determinations were carried out in a conventional temperature-programmed reduction system [27]. The reactor was constructed according to the suggestions made by Cvetanovic and Amenomiya [28]. It was placed in close contact with a brass cylinder heated by an electric oven. The temperature was controlled by a temperature-programmed controller.The gas mixture used was 4.5% H2 in N2 at a flow-rate (F) of 30 cm 3 min -I. This mixture was dried in a molecular-sieves trap and purified over an
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oxitrap. The heating rate (/3) was 10°C min -l. The amount of catalyst used (75--400 mg) depended upon the copper loading so as to keep the ratio t3So/FCo constant [29] (So are moles of copper and Co is hydrogen concentration in moles per cubic centimeter). The analyses were performed by using a conductivity detector. Continuous voltages from the detector cell and reactor thermocouple were converted to digital signals, amplified with a data-acquisition workstation and stored in a PC. 2.2.5. Electron p a r a m a g n e t i c resonance
EPR measurements were made at room temperature on a Bruker spectrometer operated at X-band frequencies. 9.8 GHz Klystron frequency, 100 kHz magnetic field modulation and 10 Gauss/s scan rate were used. The spectrometer was equipped with an on-line computer for data processing. The catalysts were placed in quartz tubes of 4 mm (o.d.), on a bed of quartz-wool, After adequate pretreatment the tubes were sealed with a torch flame. A sealed tube with just the quartz-wool bed was used as blank. 2.2.6. Nitrous oxide chemisorption
In order to determine the copper surface exposed and the dispersion in the reduced samples, the nitrous oxide chemisorption was used. The procedure employed is based on measurement of the hydrogen consumption during temperature-programmed reduction (TPR) after complete bulk oxidation, X, and alter surface oxidation of the same catalyst sample, Y. After complete oxidation, hydrogen consumption provides a measure for the total amount of copper in the catalyst, whereas hydrogen consumption after surface oxidation provides a measure for the number of copper surface atoms. This procedure was, first, proposed by Bond and Namijo [18] and then, it was completed by van der Grift et al. [19]. The last authors also studied the effect of temperature and time for the passivation step and they concluded that copper surface saturation by adsorbed oxygen could be established rapidly by N20 decomposition at temperatures between 90 to 120'C. In our case, surface oxidation (passivation) was performed by dissociative adsorption of nitrous oxide (5% N20/N2) onto the reduced catalyst at 90cC for 20 min. Complete oxidation of the catalyst was carried out with a mixture of 20% O2/N2 at 450~C for 60 min.
261
2.2.7. X A N E S
The XANES experiments were performed by using synchroton radiation at beamline X-18B of the National Synchroton Light Source, Upton, NY. The sample (IE3) was ground with a mortar and pestle and compressed to form a disk which was placed in the cell. The XANES spectra were obtained by measuring the incident and transmitted X-rays in the CuK-edge region. Peaks in the XANES spectrum were found by examining the second derivative of the spectrum. The sample was subjected to a TPR experiment, in which the material was reduced under a stream of 5% H: and 95% N 2 while the temperature was increased from room temperature to 450~'C over 45 min ramp. A XANES spectrum was taken before and after the first TPR peak and eight times during the TPR. Since the collection of a XANES spectrum took approximately 5 min, each spectrum actually represents an approximately 5 0 C range of temperatures. 2.2.8. Catalytic test
Dehydrogenation of methanol was carried out in a conventional fixed bed reactor system. The catalyst (1.0 g) was held in place by a glass-wool sandwich. A methanol/helium mixture at 17/83 molar ratio (total feed rate 30 cm 3 min 1) and 93.32 kPa pressure was used asreactant gas for all the data shown in this work. The reaction temperature was 230C. Analyses of the reaction products were performed with an on-Dine gas chromatograph Hewlett-Packard 5790 A containing a Porapak T column and a flame ionization detector. Complementary analyses were performed with a second gas chromatograph Konik (KNK-3000 HRGC) with a Porapak T column and a thermal conductivity detector. Before activity measurements, the catalysts were activated in two different conditions of time and temperatures {300'C for 60 min and 450 C lot 120 min) with a mixture of 25% HffN>
3. Results With the preparation method used in this work, about 90-95% of the copper complex was taken by the silica from the solution after a contact time of 60 h. This is different from the preparation method employed by Kohler et al. [12] in which a large volume of solution was used. They reported that the
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Table 1 Copper loading and specific surface area of fresh and used catalysts Catalyst
Cu wt%
IE0.5 IE! IE1.5 IE2 IE2.5 IE3 IE3.5
Fresh catalysts
0.45 0.84 1.33 1.71 2.15 2.51 3.01
Used catalysts
SSA (mZ/g)
Pore radius (,~)
SSA (mZ/g)
138 142 132 124 120 114 116
n.d. 76 78 82 80 n.d. 88
n.d. 137 n.d. 128 n.d. 119 114
n.d.: not determined.
ion exchange equilibrium was reached in short time, leaving a high concentration of copper in solution, The experiments of temperature-programmed decomposition (not shown here) were carried out on dried samples to follow the decomposition process of the copper ammonia complex linked to the surface of the silica. The calcination program of the catalysts was defined on this basis, SSA determinations (BET) of fresh and used catalysts are shown in Table 1. It can be observed that the
surface areas of the catalysts are only 25-30% of the surface of the original support while the same surface area was observed for both the fresh and used catalysts. The copper loadings measured by XRF gave in all cases lower concentrations than the nominal ones. The measured values of copper (wt%) percentage are given in Table 1. Fig. 1 shows the XRD diagrams of WI3 and IE3.5 catalysts, both calcinated (fresh) and reduced (used).
**
A
I 0
I
I 20.
I
a-
I 40
~
e"
I
/' ~
0
20
I
I
60
40
,
b-
60
~
I 80
80
C-
0
I
0
20
,
I
20
40
,
I
60
,
40
f
60
80
,
I
,
80
20 (degree) Fig. 1. X-ray diffraction patterns of: (a) calcinated (unreduced) WI3 catalyst; (b) used WI3 catalyst; (c) calcinated (unreduced) IE3.5 catalyst and (d) used IE3.5 catalyst. • CuO ;* Cu °.
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
The IE3.5 catalyst, both fresh and used, did not show signal either of metallic copper or of copper oxide (CuO). The IE3.5 catalyst pre-reduced by hydrogen but not used in reaction showed an XRD spectrum identical to that of the used one (Fig. 1(d)). In the case of WI3 catalyst, the typical pattern of CuO was observed in the fresh unreduced catalyst and the Cu ° pattern in the used one. It is commonly accepted that adsorptive decomposition of nitrous oxide is a successful method for determining the free-copper surface area of supported and non-supported copper catalysts. The dissociative absorption proceeds according to the reaction: N20(g) + 2Cu(s) --+ Cu(s)2Oads + N2(g), (1) where Cu(s) denotes a copper surface site. Several analytical techniques have been proposed [30-34] for monitoring the course of the reaction. In this paper we used the procedure described by Bond and Namijo [18] and van der Grift et al. [19] in which copper surface determination is based on measurement of hydrogen consumption after complete oxidation of the catalyst, X, and hydrogenconsumptionafter surface oxidation, Y, of the catalyst. The total amount of reducible copper is measured by means of X (Eq. (2)). The number of copper surface atoms is measured by Y (Eq. (3)), assuming an adsorption stoichiometry of one oxygen atom per two copper surface atoms (Cu~-O-Cus) in order to calculate the number of copper surface atoms from the amount of dissociated nitrous oxide [30,32,35]. Bulk plus surface copper reduction CuO + H2 --+ Cu + H~O hydrogen consumption = X (2)
263
planes gives an average copper surface atom area of 0.0711 nm 2, equivalent to 1.4× 1019 copper atoms per square meter. By assuming a spherical shape of the copper metal particles, S and d .... can be expressed as Eqs. (5) and (6), respectively S = 2 . Y . N a v / ( X " Mcu • 1.4 x 1019) ~- 1 3 5 3 Y / X (m2Cu/gCu), dv.s = 6 / ( S . Pcu) ~ 0.5 • X / Y (nm)
(5) (6)
with Nav=Avogadro's constant, Mcu=relative atomic weight, pcu=copper density--8.92 (g/cm3). Fig. 2 illustrates the typical hydrogen consumption profiles of the completely oxidized and surface-oxidized catalysts (IE1.5 and IE3.5). The second TPR carried out to determine Yafter dissociative adsorption of nitrous oxide showed that hydrogen consumption began at a lower temperature than that of reduction of the completely oxidized particles. This indicated that a new type of oxidized species was being reduced. Dispersion (D), specific copper surface area (S) and average volume-surface diameter of copper particles (dv.~.) obtained for all the catalysts are given in Table 2. The values of dv.~. calculated from the experimental measurements were very small (1.32 to 2.27 nm). Despite the rather good reproducibility of the method, the measurements of D and S did not follow the expected tendency of increase D and S as copper loading decrease. But they showed increasing values of D and S as catalysts loading increased up to IE2. Then, D and S remain almost constant. Complementary experiments were performed with IE1.5 and IE3.5 catalysts. In both cases, the first TPR were stopped at the valley between the two reduction peaks
Surface copper reduction only Cu(s)2Oads q- H2 --~ 2Cu(s)
--H20 hydrogen consumption = Y
(3)
Table 2 Dispersion (D), Specific Copper Surface Area (SI and Average Volume-Surface Diameter of Copper Particles (d, ~.) as a function
Hydrogen consumption (X,Y) can be used to calculate the dispersion (D), the specific copper surface area (S) and the average volume-surface diameter (dv.s.) by means of the following assumptions and equations: D = 2. Y/X (4)
of copper loading
The area per copper surface atom in the (100), (110) and (111) planes are 0.065, 0.092, and 0.0563 nm 2, respectively [36]. An equal abundance of these three
Catalyst
D
S (m 2 Cu/g Cu)
IE 0.5 IEI IEI.5
0.44 0.60 0.60
298 406 406
dw. (nm) 2.27 1.66 1.66
IE2
0.76
514
1.32
IE2.5 IE3
0.74 0.74
501 501
1.35 1.35
IE3.5
0.75
507
1.33
264
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
/.~ IE3.5/
30
700
,% /
25
20
*" 600
o~
~ 400
a ,o "J
~
g,
0
20
U"/.
•
~
I
~_
200
•
40
60
80
100
time (rnin)
Fig. 2. Hydrogenconsumptionprofiles of the completelyoxidized (filled symbols) and surface-oxidized(open symbols). ( 0 0 ) IEI.5 and ([S]m) 1E3.5 catalysts.
(referred to as point V) and afterwards, they were passivated with N20. Surface oxidation was not detected for IE1.5 whereas for the IE3.5 some surface oxidation was clearly detected, Temperature-programmed reduction (TPR) has been extensively applied in recent years for characterizing reducible catalysts including metal and metal oxide systems. This technique, therefore, allows a profile or 'finger print' of catalyst reduction to be obtained. This is eminently suitable for studying low loading highly dispersed systems whose characteristics are beyond the limits of detectability by most other direct structural analysis methods (eg. X-ray diffraction). Figs. 3 and 4 depict the TPR spectra obtained over the catalysts with different copper loading. Two peaks are seen with maxima at nearly 250350°C (referred to as peak I) and 600-650°C (peak II). Each peak area can be evaluated by decovolutionintegration of the TPR profile, namely, A1 and An. The low loading catalysts (IE0.5, IE1, IE1.5, IE2) showed AI'~AII while the high loading catalysts (IE2.5, IE3, IE3.5) gave AI clearly bigger than An. As a comparison, TPR of a Cu/SiO2 catalysts prepared by impregnation (WI3) and CuO bulk were carried out. In both cases, there appears only one peak close to peak I (Fig. 4).
Taking into account hydrogen consumption by means of the response calibration performed before and after each TPR, it can be observed that the copper of the impregnated catalyst (WI3) was completely reduced. In contrast, the copper of the IE catalysts was only reduced up to between 85-95%. The IE1.5 and IE3.5 catalysts dried and partially reduced up to different stages were studied by EPR. The partial reduction stages were carried out in the specially modified TPR equipment. The catalysts were treated under the same conditions as TPR expefiments. When the desired level of reduction was reached (determined by monitoring the hydrogen consumed), the flow of the reduction mixture was stopped, purged with nitrogen and the tube was carefully sealed. The results of EPR measurements are given in Fig. 5. The 5a and 5b spectra correspond to dried IE1.5 and IE3.5 catalysts respectively. They showed strong signals in the perpendicular region of the field approximately proportional to copper loading. The IEI.5 catalyst showed values of g//=2.263, A//=178.5 G and g±=2.036 (A± unresolved). The spectra of IE3.5 catalyst could be interpreted as having two types of Cu 2+ ions. The new copper species could be characterized by g//=2.28, A//=187 G and g±=2.06 (A± unresolved). The 5c to
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
265
700
20
~
600
/ /
15
~'''"~
500 G
•1-
o.._..
400 :~E
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10
.
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,
,
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100 _
.
0
I 40
20
,
I 60
' %
~
; '~J 80
0 100
t i m e (rain) Fig. 3. Temperature programmed reduction patterns for IE0.5, IEI, [El.5 and IE2 catalysts.
30
W13 ( x 0.5)
7oo
60O
/
25
'~
IE3.5
/r cu°
,,
20
400 ~
E
IE3
c- 15 o
IE
=
300 /
~
j/.,
IE3.5
E
200 ./
IE2.5
5 ~
100
0
0 0
20
40
60
80
100
time (min) Fig. 4. Temperature programmed reduction patterns of IE2.5, IE3, IE3.5 and WI3 catalysts and of bulk CuO.
5h spectra correspond to samples in different stages of reduction. The 5c and 5d spectra performed with the samples partially reduced up to the maximum of peak I (see Figs. 3 and 4) showed a strong decrease of Cu 2+ signal relative to the dried samples. This effect was higher for the IE3.5 than for the IE1.5 catalyst. The
5e and 5f spectra carried out with samples reduced up to point V between the two peaks, showed nearly negligible signals of cupric ions. Finally, the 5g and 5h spectra performed with catalysts reduced to the end of hydrogen consumption, showed no signal of Cu 2+.
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
266 I
_
'
I
'
I
IF1.5
I
_
a-
I
X 15
2000
i
X30
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3000
,
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X20
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X20
y__
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I
4000
2000
7,20
,
I
3000
,
I
4000
gauss (H) Fig. 5. Electron Paramagnetic Resonance spectra of IE1.5 and 1E3.5 catalysts after different pretreatments. (a, b) dried at 100°C for 7 h, (c, d) reduced up to the maximun of peak I heating at 10°C min -1, (e, f) reduced up to point V (between peak I and peak II) heating at 10°C min -I, (g,h) reduced up to the end of hydrogen consumption heating at 10°C min -1.
In addition, a complementary EPR measurement was carried out with an IE3 catalyst completely reduced and treated with N20 for 20 min at 90°C. No signal of Cu 2÷ was observed.
The XANES spectrum of the IE3 catalyst before reduction resembles that of the CuO standard, having peaks at 8977, 8986, 8993 and 8998 eV. The 8977 and 8998 eV peaks have been attributed to ls---~3d and
267
E.D. Guerreiro et al./Applied Catalysis A." General 165 (1997)259-271
1S-+4p transitions, respectively, in other copper (II) c o m p o u n d s [37]. D u r i n g the T P R e x p e r i m e n t the X A N E S spectrum changes appreciably (Fig. 6) with
the original peaks at 8977, 8986 and 8998 eV decreasing and new peaks at 8982 and 9003 eV increasing. The greatest change occurs at ~300c'C temperature
3,0-
~ 2.0,.,
tlllf',\
N
~1.0 0.5
°
_
0,0 8940
8960
8980
I 9020
9000
I 9040
I 9060
L 9080
Energy (eV)
Fig. 6. Normalized XANES spectra of [E3 catalyst taken before, during and alter the first TPR peak.
1 O0
i
,
,
i
,
,
i
,
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1.5
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Cu- Loading (wt%) Fig. 7. Conversion of methanol (filled symbols) and selectivity to methyl formate (open symbols) as a funtion of copper loading. Activation pretreatment with a mixture of 25% H2/N2: (Fq, II) for 1 h at 300'C, (A, A) for 2 h at 450' C.
268
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
range. The peak at 8982 eV is assignable to a l s ~ 4 p transition in Cu(I). Thus, it appears that under the TPR conditions a Cu(II) species is being reduced to a Cu(I) species. Fig. 7 shows the results of methanol conversion (XM) and selectivity to MF (SMF) obtained at 230°C for catalysts with different loading. The activities of the catalysts with lower copper content (IE0.5 to IE2) activated by reduction at 300°C were clearly smaller than those obtained after reduction at 450°C. In contrast, catalysts with higher copper content (IEI.5 to IE3.5) showed very small difference when they were activated by reduction at 300°C or at 450°C. With a higher copper content catalysts it was possible to obtain XM values of about 50-55%. The SMF values (60-65%) were not affected clearly by the activation conditions, In addition, further experiments were carried out on the basis of the information obtained. The reactor used to measure the catalytic activity was adapted to perform in-situ TPR experiments up to different temperatures, and immediately after that to evaluate the catalytic activity. A thermal conductivity detector was used to monitor hydrogen consumption. The methanol conversions obtained with the catalysts IE1 and IE1.5 after reduction up to point V were very small (3-4%) but for IE3.5 it was remarkably higher (X=18%). When the same catalysts (IE1, IE1.5 and IE3.5) were reduced until after peak II and then, evaluated, they showed higher conversions of 22, 29 and 60% respectively,
4. Discussion From data shown in Table 1 it can be seen that SSA of the catalysts are by far lower than the SSA of the support. The preparation method used modifies strongly the porous structure of the support. This could be due to the long-time contact with [Cu(NH3)4] 2+ solution of high pH used in order to favor ionic exchange, Beckler and White [38] showed that XRD was a sensitive tool to define completion of monolayer of different copper complexes on silica. Samples having loadings of the hexanuclear copper complex just 2% in excess of a monolayer showed the presence of peaks characteristic of the polycrystalline metal complex
[38]. With the 3% Cu/SiO2 prepared by wet impregnation (WI3) and used as comparison, the typical CuO pattern was observed when it was unreduced and the Cu ° pattern when it was used. The XRD diagrams of calcinated IE catalysts (unreduced) did not show the typical CuO diffraction lines, thus, indicating that crystals large enough to be observed by XRD were not present, even in the high loading catalysts. The reduced catalysts both fresh and used did not show any signal of metallic copper. This indicates that the copper particle size was very small or amorphous and that sintering process during reaction, if any, was not very important. The values of dv.s. obtained from chemisorption of N20 were very small and are in agreement with the XRD results. The values of D were higher than most of the copper supported catalysts but, in general, they are in agreement with the dispersion mentioned in bibliography for catalysts prepared through similar method [1]. The lower D and S values of the low loading catalysts is not in agreement with the expected trend. It is understandable that catalysts with copper loadings exceeding the ionic exchange capacity of the support (IE2.5, IE3 and 1E3.5) could show, within experimental error, small changes in the D and S values. However, the observed trend of the catalysts with copper loadings below the assumed limit of ionic exchange of the same support is not easy to be explained. It is important to mention that Kohler et al. [12] could not draw meaningful conclusions by using nitrous oxide decomposition to measure copper surface areas of Cu/SiO2 catalysts prepared by ion exchange technique. Moreover, Kenvin and White [39] have reported that highly dispersed 3.8% wt Cu/SiO2 catalyst, prepared with acetylacetonate copper complex, did not react with N20. They speculate that the catalyst does not have ensembles of Cu atoms which are close enough to react with N20 molecules. In this way N20 does not account for isolated Cu. This effect could explain our results considering that part of the copper present in our low loading catalysts could be in similar conditions. In this case the measured dispersions could be underestimated and the values of dv.s. could be overestimated. Trying to get some more evidence about this possibility, a complementary experiment was performed during a catalytic test of IE1.5. In this experiment the catalyst was reduced in H2 at 450°C for 2 h. The methanol feed was then
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
started in the usual way. After 2 h under reaction conditions, the methanol feed was stopped and the temperature of the reactor was lowered to 90°C under nitrogen flow. Then, the oxidation of the copper surface (passivation) with N20 was performed in the same conditions used for dispersion measurements. The reactor was then heated until the reaction temperature under nitrogen flow and the methanol feed restarted. After this treatment, the IE1.5 catalyst showed an initial conversion of approximately 2030% of that measured when the methanol was stopped. After 30rain under these conditions the original activity was fully recovered. This complementary experiment suggests that a fraction of the metallic surface copper is not oxidized by the N20. As Kenvin and White [39] speculate, N20 needs a minimum distance between copper ensembles. If this is so, the lower the catalysts loadings the higher the error, This would explain our unexpected results. In Fig. 7, it can be seen that methanol conversion increases with copper loading. In addition, it is a function of the activation treatment specially for low loading catalysts which require stronger activation conditions to reach their maxima activities. This explains why some low loading catalysts have been reported as low activity catalysts when they have not been properly reduced. The MF selectivities did not show any dependence on the copper loading or the activation conditions, EPR measurements on samples properly conditioned were done in order to follow the evolution of the paramagnetic Cu 2+ species during the reduction of the catalysts. Both IE1.5 and IE3.5 dried catalysts showed strong signals in the perpendicular region of the field approximately proportional to the copper loading. The IE1.5 catalyst showed values of Hamiltonian parameters corresponding to C u 2+ ion in a distorted octahedral crystal field, probably tetragonal [ 14]. The spectra of IE3.5 catalyst could be interpreted as having two types of Cu 2+ ions. The Hamiltonian parameters of this additional species could indicate the presence of Cu 2+ ions coordinated with ammonia and water molecules. Both IE1.5 and IE3.5 catalysts reduced to different levels showed similar shape of the spectra but different intensities. Both of them showed a nearly negligible signal of Cu 2+ when they were previously reduced up to point V.
269
It is clear from the TPR results that the species of copper present in the same support are strongly affected by the loading and preparation methods. The reduction of cupric ions has been studied on different kinds of catalysts by a number of authors. Depending on the support, loading, preparation method and experimental conditions, the TPR presents one, two or three peaks in a wide temperature range (230-800°C). Van der Grift et al. [40], working with catalysts prepared by IE on silica (Aerosil 380 m2/g) did not observe hydrogen consumption at temperatures over 330°C, either with catalyst of 13 wt% or with catalysts containing only l wt % copper. In contrast, Shimokawabe et al. [16] reported that reduction of Cu/SiO2 catalysts prepared by IE to show three peaks at 265-300 (c0, 440 (3), and 650800°C (7). Hydrogen consumption at these temperatures was ascribed to reduction of highly dispersed CuO clusters (c0, isolated Cu(II) ions strongly interacted with de silica surface (c~ y 7), and isolated Cu(II) ions weakly interacted with the support (c~ y 3). When studying Cu/'7-alumina catalysts Baiker et al. [41] identified two types of immobilized cupric species. By using TPR they concluded that the species had very different reducibilities. Two peaks were seen. In both cases it was considered that Cu 2 t was reduced. In agreement with the above assignment, Sermon et al. [42] who researched on the nature of Cu 2+ ions supported upon SiO2, AIO3 and TiO2 using EPR, XPS, XRD, and TPR concluded that divalent copper is stabilized in two different forms, one highly dispersed (with properties detectable by XPS and EPR) and the other poorly dispersed characterized by XRD. Similar results obtained with EPR before and after temperature-programmed reduction of a sample to point between the TPR peaks led the authors to conclude that these peaks do not reflect reduction of Cu 2+ to Cu ~+ and of Cu 1+ to Cu ° but to a reduction of Cu 2+ in two different states to essentially the zerovalent metal. As for zeolite (Cu2+,Na)-X-50 Gentry et al. [43] observed two reduction processes which were assigned to Cu 2+ to Cu -~ reductions in supercage (280~C) and sodalite cage (380°C) positions. As for (Cu2+,Na)-X-20 and (Cu2+,Na)-Y-68, Mahoney et al. [44] and Jacobs et al. [45], respectively, observed two processes assigned to reduction of Cu 2+- to Cu ~ and Cu ~+ to Cu °. The same interpretation was made by Rouco [46] by using other Cu containing catalysts
270
E.D. Guerreiro et al./Applied Catalysis A: General 165 (1997) 259-271
prepared by following the incipient wetness procedure on ~/-alumina (Condea Chemie, 190 me/g) and silica (Davison, 340 me/g), At this point, it is important to pay attention to the oxidation state of the active site in these catalysts for the dehydrogenation of methanol to MF, which is still controversial. Guerrero-Ruiz et al. [5], Tonner et al. [2] and Sodesawa et al. [4] working on different Cu/ SiOe catalysts and Matsuda et al. [24] working with Cu/TSM (fluoro tetrasilicic mica) and Cu/laponite catalysts, suggested that metallic copper is the active site. In contrast, Morikawa et al. [25] studying Cu/ TSM prepared by ion exchange and recently Domokos et al. [26] studying Cu/Zr alloys concluded that Cu 2+ ions were responsible for the dehydrogenation of methanol to ME In our case, all the fresh oxidized catalysts studied did not produce MF even after three hours under reaction conditions. All of them showed clear signals of Cu 2+ by EPR at this state. It is clear from these results that Cu 2+ was not the active site for MF formation. Both IE1.5 and IE3.5 showed a nearly negligible signal of Cu e+ by EPR when they were reduced up to point V. At the same level of reduction, the IE3 catalyst studied by XANES showed that the predominant species was Cu +. For low and medium loading catalysts (IE0.5, IE1, IE1.5 and IE2) reduced up to point V, practically no reaction was observed with NeO indicating that the amount of metallic copper was not enough to be detected by this method. The methanol conversions measured with IE1 and IE1.5 catalysts reduced up to point V were very small (3-4%). In addition, the color of the catalysts at this stage was almost white which is characteristic of Cu ÷. From the above comments peak I of the low and medium loading catalysts could be assigned to the reduction of Cu 2+ to Cu ÷. Since the peak areas of these catalysts were very close (AI~An), peak II should be assigned to the reduction of Cu + to Cu °. Taking into account that for IE1 and IEI.5 catalysts reduced up to the end of hydrogen consumption (almost black color) the methanol conversions were 22% and 29%, respectively, it becomes clear that metallic copper was the active site. Moreover, when the IE1.5 catalyst was passivated with N20 during a catalytic test, producing surface oxidation of Cu ° to Cu +, the conversion of methanol was drastically lowered, confirming the previous conclusion,
For high loading catalysts (EI2.5, EI3 and EI3.5) the TPR patterns were similar to those of low and medium loading, but AI was bigger than An. These catalysts reduced up to point V showed an appreciable reaction with N20, and the methanol conversion of IE3.5 measured at that point was 18%. These facts indicate that some amount of metallic copper was present after the first reduction peak. When the IE3.5 catalyst was reduced up to the end of hydrogen consumption, the methanol conversion was 5760%. At this condition, the reaction with NeO was clearly increased. The behavior of high loading catalyts could be explained assuming the presence of other copper species ($2) which reduce from Cu 2+ to Cu ° under peak I. The abundance of Se depends on copper loading and it becomes evident in IE2.5 and over.
5. Conclusions 1. The ion exchange method used in this work affects the texture of the silica by strongly decreasing the SSA. 2. We propose the presence of, at least, two different copper species. The relative abundance of them is a function of the copper loading of the catalysts. Species Sl: It is highly dispersed and strongly interacts with the silica surface. It reduces in two steps (Cu e+ to Cu + and Cu + to Cu °) and in two different temperature ranges. It is abundant in low and medium loading catalysts. Species $2: It weakly interacts with the silica surface. It reduces in only one step (Cu e+ to Cu °) in a temperature range very close to that of Cu 2+ to Cu + reduction of the species $1. Its presence is evident in high loading catalysts. This conclusion is in agreement with the results of Shimokawabe et al. [ 16] for low loading catalysts. For high loading catalyst (IE3.5) we did not observe the/~ peak (440°C). 3. The dissociative chemisorption of N20 is not a proper method to measure the copper surface area on this type of catalysts prepared by ionic exchange and characterized by its high dispersion. 4. The active site for methanol dehydrogenation to methyl formate in this kind of catalysts is Cu ° instead of Cu 2+.
E.D. Guerreiro et al./Applied Catalysis A: General 165 (19971 259-271
Acknowledgements Financial support by C O N I C E T and Universidad Nacional de San Luis are gratefully acknowledged. E.D.G. thanks to CONICET for his fellowship. The authors would like to thank Dr. G. Larsen of University of Nebraska at Lincoln for the XANES measurements, A.M. Gennaro of INTEC for EPR measurements and M.E. Fixman for participating in the TPR determinations.
References [1] T. Sodesawa, React. Kinet. Catal. Lett. 24 (19841 259. [2] S.E Tonner, D.L. Trimm, M.S. Wainwright, Ind. Eng. Chem. Prod. Res. Dev. 23 (19841 384. [3] M. Ai, Appl. Catal. 11 (19841 259. [4] T. Sodesawa, M. Nagacho, A. Onodera, E Nozaki, J. Catal. 102 (19861 460. [5] A. Guerrero-Ruiz, I. Rodriguez-Ramos, J.L,G. Fierro, Appl. Catal. 72 (19911 119. [6] C. Minochi, H. Kobayashi, N. Takesawa, Chem. Lett., (19791 5(17. [7] H. Kobayashi, N. Takesawa, C. Minochi, J. Catal. 69 (19811 487. [8] D.M. Monti, M.S. Wainwright, D.L. Trimm, N.W. Cant, Ind. Eng. Chem. Prod. Res. Dev. 24 (1985) 397. [9] G. Centi, S. Perathoner, Appl. Catal. A 132 (1995) 179. [1(1] T. Sodesawa, React. Kinet. Catal. Lett. 24 (1984) 259. [11] D.M. Monti, N.W. Cant, D,L. Trimm, M.S. Wainwright, J. Catal. 100 (19861 17. [12] M.A. Kohler, J.C. Lee, D.L. Trimm, N.W. Cant, M.S. Wainwright, Appl. Catal. 31 (1987) 309. [13] M.A. Kohler, H.E. Curry-Hyde, A.E. Hughes, B.A. Sexton. N.W. Cant, J. Catal. 108 (1987)323. [14] H Tominaga, Y. Ono, T. Keii, J. Catal. 40 (1975) 197. [15] M. Shimokawabe, N. Takezawa, H. Kobayashi, Appl. Catal. 2 (19821 379. [16] M. Shimokawabe, N. Takezawa, H. Kobayashi, Bull. Chem. Soc. Jpn. 56 (19831 1337. [17] M. Amara, M. Bettahar, L. Gengembre, D. Olivier, Appl. Catal. 35 (19871 153. [18] G.C. Bond, S.N. Namijo, J. Catal. 118 (19891 507. [19] C.J.G. van der Grift, A.EH. Wielers, B.EJ. Joghi, J. van Beijnum, M. de Boer, M. de Boer, M. Versluijs-Helder, J.W. Geus, J. Catal. 131 (19911 178.
271
[20] N. Takesawa, H. Kobayashi, A. Hirose, M. Shimokawabe, K. Takahashi, Appl. Catal. 4 (1982) 127. [21] N. Takezawa, H. Kobayashi, Y. Kamegai, M. Shimokawabe, Appl. Catal. 3 (1982) 381. [22] M. Shimokawabe, N. Takezawa, H. Kobayashi, Appl. Catal. 2 (19821 379. [23] A. Baiker, D. Monti, A. Wokaum, Appl. Catal. 23 (19861
425.
[24] T, Matsuda, K. Yogo, Ch. Pantawong, E. Kikuchi, Appl. Catal. A 126 (19951 177. [25] Y. Morikawa, Adv. Catal. 39 (19931 303. [26] L. Domokos, T. Katona, A. Molnfir, A. Lovas, Appl. Catal. A 142 (19961 151. [27] D. Monti, A. Baiker, J. Catal. 83 (19831 323. [281 R.J. Cvetanovic, Y. Amenomiya, in: D.D. Eley, H. Pinesand, EB. Weisz (Eds.), Advances in Catalysis, Academic Press, Vol. 17, p. 103, New York, 1967. [29] E Malet, A. Caballero, J. Chem. Soc. Faraday Trans. 1 84(7) (19881 2369. [30] J.J.F. Scholten, J.A. Konvalinka. Trans. Faraday Soc. 65 (19691 2465. [31] B, DvorS.k, J. Pasek, J. Catal. 18 (19701 1(/8. [32] J.W. Evans, M.S. Wainwright, A.J. Bridgewater, D.J. Young, Appl. Catal. 7 (19831 75. [33] G.C. Chinchen, C.M. Hay, H.D. Vandervell, K.C. Waugh, J. Catal. 103 (19871 79. [34] E, Giamello, B. Fubini, P. Lauro, A. Bossi, J. Catal. 87 (19841 443. [35] M.J. Luys, P.H. van Oeffelt, W.G.J. Brouwer, A.P. Pijpers. J.J.E Scholten, Appl. Catal. 46 (19891 161. [36] G.L. Kington. J.M. Holmes, Trans. Faraday Soc. 49 (19531 417. [37] S.K. Pandey, A.R. Chetal, ER. Sarode, Phys. B 172 11991) 324. [38] R.K. Beckler, G. White Mark, J. Catal. 102 (19861 252. [39] J.C. Kenvin, M.G. White, J. Catal. 13(/(19911447. [40] C.J. van der Grift, A. Mulder, J.W. Geus, Appl. Catal. 60 (19901 181. [41] A. Baiker, D. Monti, A. Wokaun, Appl. Catal. 23 (19861 425. [42] P.A. Sermon, K. Rollins, EN. Reyes, S.A. Lawrence, M.A. Martin Luengo, M.J. Davies, J. Chem. Soc., Faraday Trans. 1 83 (19871 1347. [43] S.J. Gentry, N.W. Hurst, A. Jones, J. Chem. Soc.. Faraday Trans. 1 75 (19791 1688. [44] E Mahoney, R. Rudham. J.V. Summers, J. Chem. Soc.. Faraday Trans. 1 75 (19791 314. [45] EA. Jacobs, J.E Linart, H. Nijs, J.B. Uytterhoeven. J. Chem. Sot., Faraday Trans. 1 73 (19771 1745. [46] A.J. Rouco, Appl. Catal. A 117 (19941 139.