Preparation of Coimpregnated Cu− Zn on Zn-Modified Silica: Influence

Jun 9, 2010 - In this paper, we showed that the addition of zinc on silica support by impregnation with zinc nitrate or chloride (10 wt % Zn) followed...
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J. Phys. Chem. C 2010, 114, 11140–11147

Preparation of Coimpregnated Cu-Zn on Zn-Modified Silica: Influence of the Basicity of the Support and of the Nature of Zinc Species on the Catalyst Properties Ste´phanie Catillon-Mucherie, He´le`ne Lauron-Pernot, and Catherine Louis* Laboratoire de Re´actiVite´ de Surface, UMR 7197 CNRS, UniVersite´ Pierre et Marie Curie-UPMC, 4 place Jussieu, 75252 Paris Cedex 05, France ReceiVed: January 11, 2010; ReVised Manuscript ReceiVed: May 19, 2010

In this paper, we showed that the addition of zinc on silica support by impregnation with zinc nitrate or chloride (10 wt % Zn) followed by calcination at 450 °C modifies the properties of the oxide surface and induces different basic properties as determined by the test-reaction of conversion of methylbutynol. Basicity depends on the zinc species formed during calcination, zinc silicate of hemimorphite-type (Zn4Si2O7(OH)2 · H2O) for the zinc nitrate precursor and isolated grafted zinc species ((tSiO)2ZnCl(O), with (O) ) OH or H2O) for the zinc chloride precursor. Basicity of the support favors the formation of hydroxy-compound during further impregnation with copper or copper-zinc nitrate. The dispersion and crystallinity of these precursors directly govern the final state of the catalyst after reduction, i.e., the copper particle size and the extent of the Cu0-ZnII interactions; the latter is attested through the test-reaction of selective hydrogenation of crotonaldehyde. I. Introduction In a former study on Cu/SiO2 catalysts prepared by impregnation to incipient wetness of silica with Cu(NO3)2 · 3H2O,1 we showed that the drying temperature of the sample after impregnation has a drastic influence on the nature of the supported copper phase and on the size of the copper metal particles obtained after reduction. After drying at room temperature (RT) in air, copper is an amorphous copper nitrate (blue color), whereas drying at 90 °C in air induces the transformation of Cu nitrate into Cu hydroxynitrate (Cu2(OH)3(NO3)), which is green-blue. After thermal reduction under hydrogen, the copper particles are smaller in the sample previously dried at RT than those dried at 90 °C. In other words, the formation of hydroxynitrate is detrimental for the formation of small metal particles. According to another study on Zn/SiO2,2 we showed first that the impregnation of silica with Zn(NO3)2 · 6H2O, followed by a drying step at RT and a calcination treatment at 450 °C, leads to the formation of zinc silicate of hemimorphite-type (Zn4Si2O7(OH)2.H2O), and that ZnO does not form at all. Only when drying is performed between 90 and 150 °C are traces of ZnO detected after calcination. The formation of zinc oxide could be related to that of zinc hydroxynitrates during the drying step in this range of temperature. These two studies pointed out the drastic influence of the drying step after impregnation on the final state of the samples after oxidation or reduction thermal treatment, and revealed a “memory effect” of the drying step on the final state of the samples, although drying was performed at much lower temperature than the subsequent thermal treatments. When mixed copper-zinc catalysts supported on silica were prepared by coimpregnation of their nitrate salts, again we observed a memory effect of the drying step.3 As in the case of monometallic Cu or Zn systems, after drying at 25 °C, the nitrates are still present on silica as an amorphous compound, while drying at 90 °C leads to the formation of an ill-crystallized * To whom correspondence [email protected].

should

be

addressed.

E-mail:

hydroxynitrate, which has been identified by X-ray diffraction (XRD) and derivative thermogravimetry (DTG) as a mixed copper-zinc hydroxynitrate, Cu2-xZnx(OH)3(NO3), in which the two cations are closely associated. After reduction at 350 °C, the mixed hydroxynitrate leads to a better interdispersion of Cu0 and ZnII in the reduced material than in the samples dried at 25 °C. This is attested by the higher selectivity to crotyl alcohol in the reaction of hydrogenation of crotonaldehyde. As in the case of the Cu/SiO2 system, smaller metallic copper particles are obtained when the samples are dried at RT than when they are dried at 90 °C, and this was again related to the formation of hydroxynitrate during the drying step. In addition, when a calcination step is performed before the reduction, the intermediate formation of CuO particles partially destroys the interaction between CuII and ZnII created during the drying step. As it was shown that the formation of mixed hydroxynitrate was a key parameter that governed the final catalyst properties, we decided to investigate the influence of the preliminary impregnation of zinc on silica before impregnation with Cu or Cu-Zn solutions. This preimpregnation may modify chemical properties of the support such as basicity, and thus its ability to transform the nitrate precursors into hydroxynitrate species. Two zinc precursors have been used: zinc nitrate, which leads to hemimorphite-type zinc silicate and another one, ZnCl2. XRD is used to follow at each step of the preparation the nature of the crystallized phases formed on the surface, and the final particle size of the materials that is a key parameter for catalytic studies is determined by transmission electron microscopy (TEM). Direct reduction of the Cu or Cu-Zn catalysts without intermediate calcination was chosen for this study to favor Cu-Zn interaction.3 The reduction temperature of 350 °C was chosen high enough to have metallic copper, but low enough to keep zinc in the oxidation state II. According to the work of Rodrigues et al4 and our former study on Cu-Zn/SiO2 systems,3 the Cu-Zn interactions can be characterized through the test-reaction of selective hydrogenation of crotonaldehyde. Crotonaldehyde, CH3CHdCH2CHdO, is an R,βunsaturated aldehyde. The first step of hydrogenation of crotonaldehyde leads to the formation of either but-2-en-1-ol

10.1021/jp100252h  2010 American Chemical Society Published on Web 06/09/2010

Basicity and Cu0-ZnII Interactions (crotyl alcohol) or butyraldehyde (butanal), the latter being both thermodynamically and kinetically favored. Earlier studies reviewed by Claus5 showed that the selectivity to crotyl alcohol can be enhanced when a metallic phase, Pt, Ru, or Co, was doped by cations (such as Fe2+ or Sn2+), or when a metallic phase, Pt or Co, was supported on reducible supports (TiO2, CeO2 or ZnO). The selectivity to crotyl alcohol, which forms via the hydrogenation of the CdO bond rather than that of the CdC bond, can be controlled by the promoting effect of a cationic agent. While, in the case of Cu/SiO2, butanal is the main product of the reaction, in the case of Cu-Zn/SiO2, the interaction between Cu0 and ZnII promotes the selectivity to crotyl alcohol, but the activity is lower for the same copper particle size.3 The reaction of conversion of 2-methylbut-3-yn-2-ol (MBOH) was used to evaluate the acid-base properties of the zincmodified silica supports. This reaction was first proposed by Lauron et al.,6 and used to characterize different kinds of catalysts.7 The reaction pathway for MBOH conversion depends on the acid-base properties of the catalysts: (i) on acidic catalysts, MBOH undergoes both dehydration and isomerization, which leads to the production of 2-methylbut-1-en-3-yne and 3-methylbut-2-enal (prenal); (ii) on basic surfaces, MBOH leads to acetone and acetylene; (iii) on amphoteric catalysts, MBOH undergoes hydration to 3-hydroxy-3-methylbutan-2-one and isomerization to 3-methylbut-3-en-2-one. II. Experimental Section 1. Sample Preparation. Preparation of Zn-Modified silica. Incipient wetness impregnation of 5 g of silica (nonporous Aerosil Degussa 380, 380 m2 · g-1 according to the manufacturer) was performed by mixing with 15 mL of aqueous solution (0.5 mol · L-1) of zinc nitrate (99.999% Aldrich), so as to achieve a Zn loading of 10 wt %. The mixture was thoroughly hand-mixed until a gel was obtained. The sample was dried in air at RT, then calcined under a flow of industrial air (Air Liquide, 100 cm3 · min-1) from RT to 450 °C with a heating rate of 7.5 °C · min-1 then at 450 °C for 2 h (ZnNO3-SiO2). The same procedure of impregnation was used for the impregnation with zinc chloride (Prolabo, Normapur, 98%). The sample was dried at 150 °C, then calcined in air in a muffle oven for 30 h at 450 °C (ZnCl-SiO2). Preparation of Cu and Cu-Zn Catalysts Supported on ZnModified Silica. Incipient wetness impregnation of 1 g of Zn/ SiO2 (ZnNO3-SiO2 and ZnCl-SiO2) was performed by mixing with 1.5 mL of aqueous solution of copper nitrate (1 mol · L-1, Rectapur, Prolabo) so as to achieve a Cu loading of 10 wt %. The samples were dried in air at RT (Cu/ZnNO3-SiO2-25 and Cu/ZnCl-SiO2-25). As a reference, a Cu/SiO2 was also prepared with 3 mL of copper nitrate solution (0.5 mol · L-1) for 1 g of silica. The sample was dried in air at RT or 90 °C (Cu/SiO2-25 and Cu/SiO2-90). The mixed Cu-Zn samples were prepared in the same way, by coimpregnation with a 1.5 mL of aqueous solution of copper nitrate (1 mol · L-1) and zinc nitrate (1 mol · L-1) for 1 g of Zn-SiO2 (Cu-Zn/ZnNO3-SiO2 and Cu-Zn/ZnCl-SiO2). As a consequence, the Cu and Zn loadings are 10 and 20 wt %, respectively. As a reference, a Cu-Zn/SiO2 was also prepared with 3 mL of a solution of copper nitrate (0.5 mol · L-1) and zinc nitrate (0.5 mol · L-1) per gram of silica. The samples were dried in air at RT or 90 °C. The samples described above were then directly reduced in the same conditions under a flow of hydrogen (10% H2 in Ar, Air Liquide, 100 cm3 · min-1) from RT to 350 °C with a heating

J. Phys. Chem. C, Vol. 114, No. 25, 2010 11141 rate of 7.5 °C · min-1 and no plateau of temperature, i.e., under the same conditions as for the TPR experiments described below. According to our former work3 under these conditions, only copper is reduced, not ZnII. 2. Techniques. The chemical analyses of the samples were performed by inductive coupling plasma in the CNRS Center of Chemical Analysis (Vernaison, France). The XRD patterns were recorded on a Siemens diffractometer (D500) using Cu KR radiation. The phase identifications were performed by comparison with the tabulated Joint Committee on Powder Diffraction Standards (JCPDS) d spacing files. Thermal analyses (DTG) of the samples were obtained on a SEIKO TG-DTA 320 module operated by a SEIKO SSC5200 disk-station. Samples (20 mg) were heated (5 °C.min-1) under air flow (100 mL.min-1). The nature of the effluents was analyzed by mass spectrometer (HPR20/DSMS from Hiden Analytical) through the profile of their ions of fragmentation (for water m/z ) 17 (OH+) and 18 (OH2+) and for nitrate ions m/z ) 30 (NO+), 44 (N2O+) and 46 (NO2+)). The sizes of the metal copper particles obtained after reduction up to 350 °C were measured from the electron micrographs obtained by TEM (JEOL 100 CXII). The size limit for the detection of copper particles on SiO2 was about 1 nm. The histograms of particle sizes were established, and the average particle diameter ds was calculated using the following formula: ds )∑nidi/∑ni where ni is the number of particles of diameter di. The extended X-ray absorption fine structure (EXAFS) measurements were performed at the Zn K edge at the XAS beamline of the synchrotron radiation facility (Orsay, France) under the same conditions as in ref 2. Data analysis was also performed according to ref 2. X-ray photoelectron spectroscopy (XPS) spectra were collected on a SPECS Phoibos100 5MCD X-ray photoelectron spectrometer, using a Mg KR (hν ) 1253.6 eV) X-ray source having a 300 W (23 mA, 13 kV) electron beam power and a 7 × 20 mm spot size. The emission of photoelectrons from the sample was analyzed at a take off angle of 90° under ultrahigh vacuum conditions (1 × 10-8 Pa). XPS spectra were collected at pass energy of 20 eV for the spectra between 0 and 1100 eV and of 10 eV for Si2p, O1s and Zn2p core XPS levels and Auger Zn. No charge compensation was applied during acquisition. After data collection, the binding energies were calibrated with respect to the binding energy of the Si2p peak at 103.3 eV. The XPS measurements were carried out at RT. 3. Catalytic Reactions. The reaction of methylbutynol (MBOH) conversion was performed in gas phase under atmospheric pressure in a flow reactor. For each experiment, 100 mg of catalyst was deposited on a porous frit in the center of a U quartz tube of 10 mm internal diameter. After an activation pretreatment under nitrogen flow (70 cm3 · min-1) at 400 °C (5 °C · min-1) for 2 h, the reactor was cooled down to the reaction temperature of 120 °C. The reaction temperature was maintained within (1 °C thanks to a thermocouple located close to the wall of the quartz tube. The desired partial pressure of MBOH (1.73 kPa) was obtained by bubbling nitrogen (100 cm3.min-1) in liquid MBOH (Fluka, 99,9%) at 20 °C, which then passed through the reactor. The reaction products were analyzed using a Perichrom PR2100 gas chromatograph equipped with a flame ionization detector (FID) and a 15% tetracyanoethylated pentaerythritol (TCEPE) on chromosorb P column. Because acetone and acetylene were the only products detected, i.e., characteristics of the basic pathway only, catalytic data were expressed in terms of conversion only. The partial

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pressure of each product Pi was calculated from chromatographic measurements by using the appropriate response coefficient and the value of the initial partial pressure of MBOH in the feed, 0 . The conversion τ is given by PMBOH

τ(%) )

1/2 · (Pacetylene + Pacetone) 0 PMBOH

The hydrogenation of crotonaldehyde was also performed under atmospheric pressure in a glass flow reactor. Fifty milligrams of catalyst was deposited on the frit of a pyrex reactor. The catalysts were activated in situ under pure hydrogen (30 mL · min-1) from 25 to 350 °C (8 °C · min-1). Then, the reactor was cooled down to the appropriate reaction temperature to maintain the conversion below 7% to limit secondary reactions and over 4% to minimize the errors. The crotonaldehyde (Fluka), used as received and thus containing 0.1-0.3% butanal, was kept at 0 °C in a saturator. The reaction was performed under a flow of crotonaldehyde (0.247 mol.s-1) in H2. The reaction products were analyzed on line by gas-liquid FID chromatography (GLC, with a 30 m long and 0.5461 mm diameter DB-Wax column (J&W Scientific) heated at 85 °C). The reaction activities (µmole · s-1 · gCu-1) were calculated according to the following formula: A ) RF/ω, where R is the crotonaldehyde conversion, F is the crotonaldehyde flow rate in moles per second, and ω is the mass of copper in grams. The selectivity to crotyl alcohol was expressed as the molar ratio of the selected product to the all products formed. III. Results 1. Characterization of the Zn-SiO2 Samples. As mentioned in the Introduction, according to a former study,2 the Zn phase in the ZnNO3-SiO2 sample dried at 25 °C then calcined at 450 °C is an amorphous zinc silicate of hemimorphite-type (Zn4Si2O7(OH)2 · H2O). ZnO does not form during calcination, even as traces. The calcined ZnCl-SiO2 sample does not show any XRD peaks that could indicate the presence of ZnO. Likewise, the UV-visible spectrum does not show the characteristic absorption band of ZnO at 350 nm, corresponding to the ZnO bandgap of 3.35 eV. The sample was therefore characterized by XAFS, and compared to the calcined ZnNO3-SiO2 sample, the EXAFS and simulation of which can be found in ref 2. The main difference between the moduli of their Fourier transform (FT) (Figure 1A) is the presence of a shoulder after the peak of the first neighbors and a narrower peak of the second neighbors for the ZnCl-SiO2 sample (curve a) with respect to calcined ZnNO3-SiO2 (curve b). The simulations of the EXAFS signal and of the FT (modulus and imaginary part) of ZnCl-SiO2 are shown in Figure 1B,C, and the parameters of the fit are reported in Table 1, where they are compared to those of ZnNO3-SiO2. The first shell fits well with the contribution of three O atoms at a distance of 1.96 Å and one Cl atom at longer distance, 2.22 Å, which is responsible for the shoulder of the first peak (Figure 1Aa). The Zn-O bond distance is equal to that in zinc hemimorphite whose zinc is tetrahedral, and is smaller than that in octahedral [Zn(H2O)6]2+ (2.08 Å,2). The second shell contains only Si neighbors, and no Zn, in contrast with the Zn hemimorphite in the calcined ZnNO3-SiO2 sample. For this reason, the peak of the second neighbor is narrower than that for zinc hemimorphite (Figure 1A). These results bring several elements of information: (i) the zinc species obtained after impregnation of ZnCl2 and calcination are tetrahedral; (ii) they do not interact with each

Figure 1. FT modulus (A) of (a) calcined ZnCl-SiO2 and (b) calcined ZnNO3-SiO2. Simulation of the first two shells of the EXAFS signal of the calcined ZnCl-SiO2 sample, (B) in the k-space (kχ(k)) and (C) in the R-space (w(k)k3χ(k)).

other; (iii) the remaining chloride in the calcined sample is located in the Zn coordination sphere; (iv) the tetrahedral zinc complex is grafted onto the silica support through two bonds. The following model for the Zn species present in calcined ZnCl-SiO2 is proposed: (tSiO)2ZnCl(O) with O belonging to an OH group or a H2O molecule. From these results, it can be concluded that the use of two different zinc precursors, Zn(NO3)2 · 6H2O and ZnCl2, for the impregnation of silica leads to two different zinc species after calcination: amorphous zinc hemimorphite and grafted zinc species, respectively. These samples were also characterized by XPS. The main difference in the spectra recorded between 0 and 1100 eV in binding energy is the presence of a small XPS Cl peak for calcined ZnCl-SiO2, which corresponds to a Zn/Cl ratio of 6. Otherwise, the binding energy of the XPS peak of Zn2p3/2 is the same for both calcined ZnCl-SiO2 and calcined ZnNO3-SiO2

Basicity and Cu0-ZnII Interactions

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TABLE 1: Best Fit Parameters of the EXAFS Spectra of the Calcined ZnCl-SiO2 and ZnNO3-SiO2 Samplesa sample calcined ZnCl-SiO2 calcined ZnNO3-SiO2

backscatterers

N

σ (Å)

R (Å)

∆E0 (eV)

O Cl Si O Si Zn

3.0 1.0 2.0 4.0 1.0 1.4

0.06 0.07 0.07 0.07 0.09 0.10

1.96 2.22 3.18 1.96 3.12 3.31

7.7 0.6 12 -1.5 -2.0 -6.0

F (%)

ref this work

3.5 2 0.9

a N: number of neighbors; σ (Å): Debye Waller factor; R (Å): distance between Zn and a backscatterer; ∆E0 (eV): energy shift; F (%): agreement factors; Standard errors in EXAFS are (0.02 Å in distance and (10% in coordination number.

Figure 2. Conversion of MBOH versus the reaction time of (a) calcined ZnNO3-SiO2, (b) calcined ZnCl-SiO2, and (c) silica support.

samples (1022.6 eV) and the kinetic energy of the Auger Zn peak as well (986.7 eV). Hence, neither the XPS peak of Zn2p3/2, nor the Zn Auger peak permits the discrimination between the Zn species in calcined ZnCl-SiO2 and in calcined ZnNO3-SiO2 samples. The Zn/Si ratio is almost the same for both samples (0.09 and 0.10, respectively), but we cannot deduce that the Zn dispersion is the same for sure in the two samples. The MBOH conversions of the calcined ZnNO3-SiO2 and ZnCl-SiO2 samples were compared to silica alone (Figure 2). As expected, silica does not show significant conversion. In contrast, the ZnNO3-SiO2 and ZnCl-SiO2 samples exhibit conversions of 90 and 35%, respectively, after 5 min of reaction. Only the products of the basic pathway were detected, i.e., acetone and acetylene. ZnNO3-SiO2 is therefore more basic than ZnCl-SiO2, and support basicity evolves as follows: ZnNO3-SiO2 > ZnCl-SiO2 > SiO2 ∼ 0. The samples deactivate during time on stream, but ZnNO3-SiO2 remains more active than ZnCl-SiO2. According to previous studies analyzed in a recent review,7 deactivation of basic sites is due to the poisoning of the active sites by the secondary products arising from acetone polymerization. 2. Characterization of the Cu/Zn-SiO2 Samples. An obvious difference between the Cu/Zn-SiO2 and Cu/SiO2 samples appears during the stage of impregnation. While the wet Cu/ SiO2 sample remains as blue as the solution of copper nitrate, the wet Cu/ZnNO3-SiO2 and Cu/ZnCl-SiO2 solids turn greenblue and green, respectively, as soon as the copper solution is added. These changes of color reveal a change in the nature of the copper species due to the presence of zinc. The color of the samples remains the same after drying at RT. It is noteworthy that Cu/SiO2 was blue after drying at RT, but turned greenblue during drying at 90 °C. According to our former study,1 this is due to the transformation of copper nitrate into copper hydroxynitrate (Cu2(OH)3(NO3)) during drying at 90 °C. The two Cu/Zn-SiO2 samples dried at RT exhibit diffraction peaks (Figure 3c,d). The diffractogram of Cu/ZnNO3-SiO2

Figure 3. XRD patterns of dried samples: (a) Cu/SiO2-90; (b) Cu/ SiO2-25; (c) Cu/ZnNO3-SiO2-25; (d) Cu/ZnCl-SiO2-25; (e) JCPDS file of Cu2(OH)3(NO3) (#15-0014); (f) JCPDS file of Cu(OH)Cl (#55-0324).

(Figure 3c) is identical to that of Cu/SiO2-90 (Figure 3a), which attests the presence of copper hydroxynitrate (JCPDS pattern 3e); Cu/SiO2-25 remains amorphous (Figure 3b). The diffraction peaks of Cu/ZnCl-SiO2 (Figure 3d) are much weaker than those of Cu/ZnNO3-SiO2 (Figure 3c); some of them correspond to peaks of copper hydroxynitrate, but new peaks are also visible. By comparison with JCPDS pattern 3f, they can be attributed to copper hydroxychloride (Cu(OH)Cl). This latter compound is green, and is therefore responsible for the green color of the Cu/ZnCl-SiO2 sample. One can deduce from the XRD results that copper hydroxynitrate, which formed in Cu/SiO2 only during drying at 90 °C, is already formed in the two Cu/Zn-SiO2 samples after drying at 25 °C. It probably forms during impregnation, as attested by the green-blue or green color of the wet samples. One can propose that the basicity of the Zn-SiO2 supports favors the formation of copper hydroxynitrate during impregnation. In the case of the Cu/ZnCl-SiO2 sample, in addition, the presence of chlorides also induces the formation of Cu(OH)Cl. After reduction up to 350 °C, Cu/ZnCl-SiO2 exhibits small metal copper particles between 1 and 8.5 nm with an average size of 4 nm, whereas Cu/ZnNO3-SiO2 exhibits a much broader distribution of size of copper particles, between 1 and 35 nm (Table 2). For comparison, Table 2 also shows, in agreement with our former work,1 that the copper particles in Cu/SiO2 dried at 25 °C are smaller (1-5.5 nm with an average size of 3 nm) than when the sample is dried at 90 °C (1-110 nm). 3. Characterization of the Cu-Zn/Zn-SiO2 Samples. As in the case of the impregnation with copper nitrate, the coimpregnation of ZnNO3-SiO2 or ZnCl-SiO2 with copper and zinc nitrates leads to the same changes of color of the wet

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TABLE 2: Sample Color, Nature of the Supported Copper Phase after Drying, and Copper Metal Particle Size after Reduction Cu0 particle size after reduction

after drying sample

color of the drying wet sample T (°C)

Cu/SiO2

blue

Cu/ZnNO3-SiO2 Cu/ZnCl-SiO2 Cu-Zn/SiO2

green-blue green blue

Cu-Zn/ZnNO3-SiO2 green-blue Cu-Zn/ZnCl-SiO2 a

green

25 90 25 25 25 90 25 90 25 90

color

nature of the identified supported phase a

blue green-blue green-blue green pale blue green-blue green-blue green-blue green green

amorphous Cu(NO3)2.3H2O Cu2(OH)3(NO3) Cu2(OH)3(NO3) Cu(OH)Cl + Cu2(OH)3(NO3) amorphous Cu(NO3)2 and Zn(NO3)2 Cu2-xZnx(OH)3(NO3) Cu2-xZnx(OH)3(NO3) Cu2-xZnx(OH)3(NO3) + Zn(OH)(NO3) (Cu,Zn)2Cl(OH)3 (Cu,Zn)2Cl(OH)3 + hydroxynitrates (DTG)

10 nm daverage (nm) (size distribution) (size distribution) 3.0 (1.0-5.5) 4.8 (1.0-5.5) 4.1 (1.0-10) 3.9 (1.0-8.5) 3.4 (1.0-5.5) 4.6 (1.0-9.0) 5.9 (1.0-10) 7.6 (5-10) 4.0 (1.0-9.5) 5.3 (1.0-9.5)

33 (9-110) 17 (10-35) 16 (10-23) 16 (10-50) 18 (10-50)

Except when mentioned, the phases were identified by XRD. The samples may also contain amorphous nitrates, according to DTG.

Figure 5. DTG profile performed under air flow of dried samples: (a) Cu-Zn/SiO2-90; (b) Cu-Zn/ZnNO3-SiO2-25; (c) Cu-Zn/ZnCl-SiO290.

Figure 4. XRD patterns of dried samples: (a) Cu/SiO2-90; (b) Cu-Zn/ SiO2-90; (c) JCPDS file of Cu2(OH)3(NO3) (#15-0014); (d) Cu-Zn/ ZnNO3-SiO2-25; (e) Cu-Zn/ZnNO3-SiO2-90; (f) JCPDS file of Zn(OH)(NO3) · H2O (#47-0965); (g) Cu-Zn/ZnCl-SiO2-25; (h) Cu-Zn/ ZnCl-SiO2-90; (i) JCPDS file of Cu2-xZnx(OH)3(Cl) (#50-1558).

samples (green-blue and green, respectively), as during the preparation of the Cu/Zn-SiO2 samples. The colors remain unchanged after drying at 25 or 90 °C. The Cu-Zn/SiO2 reference sample prepared by coimpregnation of pure silica is blue after drying at RT and green-blue after drying at 90 °C. At the first glance, the XRD pattern of Cu-Zn/SiO2-90 (Figure 4b) shows the same diffraction lines in the range 2θ ) 5 - 30° as Cu/SiO2-90 (Figure 4a), but less resolved, and looks like that of copper hydroxynitrate (Cu2(OH)3(NO3) (JCPDS pattern 4c). In our former paper on Cu-Zn/SiO2,3 additional DTG experiments and the sample color allowed us to conclude that the compound formed on silica was a mixed copper-zinc hydroxynitrate (Cu2-xZnx(OH)3(NO3)). The XRD patterns of the Cu-Zn/ZnNO3-SiO2 samples dried at 25 or 90 °C (Figure 4d,e) shows the same features as Cu-Zn/ SiO2-90 (Figure 4b), but with stronger peak intensity, especially for the sample dried at 90 °C. The latter sample also shows

additional peaks corresponding to zinc hydroxynitrate (Zn(OH)(NO3) · H2O) (JCPDS pattern 4f), as already observed in the ZnNO3-SiO2 sample dried at 90 °C.2 The Cu-Zn/ZnCl-SiO2 samples dried at 25 or 90 °C (Figure 4g,h) exhibit a different XRD pattern, which is characteristic of a mixed copper-zinc hydroxychloride (Cu2-xZnx(OH)3Cl) (JCPDS pattern in Figure 4i) with thinner and more intense peak for the sample dried at 90 °C. The DTG profile of Cu-Zn/SiO2-90 (Figure 5a) exhibits two broad peaks at 130 and 305 °C, corresponding to the decomposition of water and nitrates from copper and zinc nitrates according to mass spectrometry (MS) analysis, and a thinner peak at 240 °C, attributed to the decomposition of mixed copper-zinc hydroxynitrate, according to our former study.3 Cu-Zn/ZnNO3-SiO2-90, which also contains a mixed copper-zinc hydroxynitrate, exhibits the same type of DTG profile (Figure 5b), but with a more intense peak at 240 °C, which may be due to a higher amount of mixed copper-zinc hydroxynitrate, also attested by the higher intensity of the XRD pattern at 2θ above 32° (compare patterns b and e in Figure 4). The DTG profile of Cu-Zn/ZnCl-SiO2 (Figure 5c) exhibits a much more intense peak at 240 °C with a shoulder at 275 °C, a broad peak between 50 and 200 °C, and maybe another weak one above 330 °C. On the basis of the previous assignments, the two last peaks can be attributed to the decomposition of water and residual nitrates, the intense and thin one at 240 °C to the decomposition of hydroxynitrate compounds. The shoulder at 275 °C can tentatively be assigned to the decomposition of the copper-zinc hydroxychloride. The high intensity of the peak at 240 °C indicates the presence of a higher amount of mixed hydroxynitrate in Cu-Zn/ZnCl-SiO2 than in the other samples, which

Basicity and Cu0-ZnII Interactions

J. Phys. Chem. C, Vol. 114, No. 25, 2010 11145 selectivity to crotyl alcohol of the mixed Cu-Zn catalysts is higher with the ZnCl-SiO2 support (46%) than with ZnNO3-SiO2 (25%). When the Cu-Zn/Zn-SiO2 samples are dried at 90 °C, the selectivity to crotyl alcohol does not increase significantly in contrast with the Cu-Zn/SiO2 sample (Figure 6B). As already developed in our previous study,3 the Cu-Zn/SiO2 samples exhibit higher selectivity to crotyl alcohol than the Cu/SiO2 sample, especially when it has been dried at 90 °C (Figure 6). This result has been related to the formation of a mixed Cu-Zn hydroxynitrate during drying at 90 °C, which evolved toward copper particles in strong interaction with zinc cations under reducing treatment.

Figure 6. Selectivities to crotyl alcohol in the reaction of hydrogenation of crotonaldehyde performed at 120 °C over (A) Cu catalysts and (B) Cu-Zn catalysts supported on SiO2, ZnNO3-SiO2, and ZnCl-SiO2, after reduction (H2 up to 350 °C) of the samples dried at 25 °C (white bars) and at 90 °C (black bars)

was not observed by XRD (Figure 4h,i). The apparent discrepancy may be explained, either by the presence of hydroxynitrate particles too small to be detected by XRD or by amorphous ones. After reduction, if one considers the samples dried at 90 °C, the Cu-Zn/ZnCl-SiO2 sample exhibits much smaller copper metal particles, between 1 and 9.5 nm with an average size of 5.3 nm than Cu/SiO2, Cu-Zn/SiO2 or Cu-Zn/ZnNO3-SiO2 (Table 2). The particles of Cu-Zn/ZnCl-SiO2 are slightly smaller when the samples are dried at 25 °C. Note that, to complement our former work in which we ascertained that, after TPR up to at 350 °C, copper is metallic and Zn is still in the oxidation state II, we checked by XRD that the peaks of hydroxynitrates of Zn and Cu have totally disappeared after reduction at 350 °C, and that the characteristics peaks of Cu0 at 2θ are visible. In the XPS spectrum of the Cu-Zn/SiO2 sample reduced at 350 °C, both the Cu2p XPS and the Cu Auger peaks confirm the reduction of CuII, while Zn is still in the oxidation state II (1022.5 eV). So, these results confirm that at 350 °C, Cu is in the metallic state, while Zn is still ZnII. 4. Reaction of Hydrogenation of Crotonaldehyde. The Cu and Cu-Zn catalysts supported on silica, ZnNO3-SiO2, and ZnCl-SiO2 were tested in the reaction of hydrogenation of crotonaldehyde at 120 °C after in situ activation under hydrogenation at 350 °C. The contact time was adjusted to ensure an activity around 1.2 µmol · s-1 · gCu-1. The selectivities to crotyl alcohol are reported in Figure 6. For the monometallic Cu samples dried at 25 °C (Figure 6A), the selectivities to crotyl alcohol of the two Cu/Zn-SiO2 samples (18 and 25%) are higher than that of Cu/SiO2 (∼6%). The higher selectivities of the two Cu/Zn-SiO2 samples may result from the presence of interaction between Cu0 particles and the Zn2+ ions, as already reported in ref 3. The higher selectivity of Cu/ZnCl-SiO2 (25%) than that of Cu/ZnNO3-SiO2 (18%) can be related to the fact that the copper particles are smaller in Cu/ZnCl-SiO2. This would therefore favor the interaction between the Cu0 particles and the Zn2+ ions of the supports. As in the case of the monometallic Cu samples, the mixed Cu-Zn samples dried at 25 °C before reduction show a higher selectivity to crotyl alcohol when the silica support is modified by zinc (Figure 6B). The selectivities of the two mixed Cu-Zn samples on Zn-SiO2 supports are higher (25 and 46%) than those of the corresponding monometallic Cu catalysts (18 and 25%). As in the case of the monometallic Cu catalysts, the

IV. Discussion 1. Support Basicity. Zinc silicate of hemimorphite-type obtained by impregnation of zinc nitrate, drying at 25 °C, and calcination at 450 °C (ZnNO3-SiO2) and grafted Zn2+ obtained by impregnation of zinc chloride, drying at 150 °C, and calcination (ZnCl-SiO2) induce basicity on silica, as indicated by the results of the reaction of conversion of MBOH (Figure 2). To explain the different reactivity of ZnNO3-SiO2 and ZnCl-SiO2, and therefore the different basicities, one can refer to the model proposed by Breuer et al.8 From calculations, it shows how a molecule of methanol can adsorb on a surface of zinc hemimorphite. Methanol is dissociated on a Zn2+-O2acid-base pair site, leading to a methoxy bonded to the Zn2+ site and a H+ bonded to O2-. This mechanism of adsorption could directly be applied to MBOH adsorbed on the ZnNO3-SiO2 sample that contains zinc hemimorphite on the surface (Figure 7a). This deprotonating adsorption step may also be considered in the case of ZnCl-SiO2, leading in both cases to the MBOH decomposition into acetone and acetylene, as usually reported on basic catalysts7 (Figure 7b). The different reactivities of ZnNO3-SiO2 and ZnCl-SiO2 in MBOH conversion could be explained by the influence of chlorine in the Zn2+ coordination sphere, as attested by the XAFS measurements. Note, however, that XPS did not allow us to discriminate the two types of Zn, probably because the technique is not sensitive enough to small changes in the electronic properties. Cl, which is electron attractor, may lower the electron density of the oxygen atoms that become less electron donors and therefore less basic. This interpretation holds, although the number of Zn sites accessible to MBOH may not be the same in the two types of samples. For Zn in ZnCl-SiO2, the XAFS analysis showed that there is no Zn as second neighbors, which makes us confident that all Zn is grafted on silica as isolated entities, i.e., 100% accessible to MBOH. For ZnNO3-SiO2, although the XPS Zn/Si ratio is the same as that for ZnCl-SiO2, we cannot ascertain that the Zn dispersion is the same. However, since ZnNO3-SiO2 is more active than ZnCl-SiO2, it is not of prime importance to have this information. 2. Hydroxy-Compound Formation. The basic character of the Zn-SiO2 supports also allows us to explain that further impregnation with copper nitrate or with a mixture of copper and zinc nitrates leads to the formation of hydroxy-compounds during the step of impregnation, as attested by the color changes of the impregnation mixture, from blue to green-blue and green (Table 2). Impregnation of silica with copper nitrate, or with copper and zinc nitrates also leads to hydroxy-compounds, but only when drying is performed at 90 °C and not at 25 °C.1,3 One can conclude that the zinc present on the modified-silica support acts as a basic reactant with the nitrates, which readily transform into hydroxy-compounds (Table 2). However, a strict

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Figure 7. Possible mechanism of MBOH decomposition on (a) zinc hemimorphite species in calcined ZnNO3-SiO2 and (b) grafted zinc species in calcined ZnCl-SiO2.

correlation with basicity is not observed since, according to DTG experiments (Figure 5), a higher amount of ill-crystallized mixed hydroxynitrates is obtained on ZnCl-SiO2 than on ZnNO3-SiO2 support. In the case of the ZnCl-SiO2 support, one can propose that the grafted isolated Zn2+ species obtained after calcination act as nucleation sites for the particles of copper hydroxynitrate and hydroxychloride and favor their formation, leading to welldispersed phases. To summarize, the basicity of the Zn-modified silica supports evaluated by MBOH conversion favors the formation of hydroxy-compounds during copper or copper-zinc impregnation, but the amount and dispersion of the hydroxy-compounds also depend on the dispersion and nature of the Zn species obtained after calcination. 3. Copper Particle Size after Reduction. If one examines the size of the copper metal particles obtained after reduction up to 350 °C of the samples dried at 25 °C (Table 2), one can note that they are always smaller when they are supported on ZnCl-SiO2 whose Zn2+ species is a grafted isolated entities, than when they are supported on ZnNO3-SiO2, whose Zn2+ species forms a zinc silicate, i.e., a polymerized-type surface compound. Formation of well-dispersed and ill-crystallized hydroxy-compounds on ZnCl-SiO2 support attested by the weaker diffraction peaks of hydroxy-compounds in Cu or Cu-Zn on ZnCl-SiO2 (Figures 3d and 4g,h) than in Cu or Cu-Zn on ZnNO3-SiO2 (Figures 3c and 4d,e), leads to smaller metal particles after reduction than on ZnNO3-SiO2. These results are consistent with the concept that the size of metal particles is determined by the size of the precursor particles, as already observed by us during the preparation of several systems (Ni,9 Cu,1,3 Au10). In agreement with our former works,1,3 it is also found that the drying temperature of 90 °C induces a better crystallization of the hydroxy-compounds and favors the formation of larger copper particles. 4. Hydrogenation of Crotonaldehyde. Higher selectivities to crotyl alcohol were obtained for the mixed Cu-Zn samples, Cu-Zn/ZnNO3-SiO2 (25-27%)andCu-Zn/ZnCl-SiO2 (46-48%), dried at 25 or 90 °C and for Cu-Zn/SiO2 (27%) dried at 90 °C than for the monometallic Cu catalysts (Figure 6). These three

catalysts contain mixed Cu-Zn hydroxynitrate and/or hydroxychloride after drying (Table 2). According to our former paper,3 the close proximity of CuII and ZnII in the supported phase after drying favors the formation of Cu0-ZnII interactions after reduction at 350 °C, and therefore the high selectivity to crotyl alcohol. To explain the higher selectivity of the Cu0-ZnII/ZnCl-SiO2 sample, the amount of Cu-Zn mixed hydroxy-compound must be considered before reduction: the surface of the DTG peak at 240 °C (including the shoulder at 275 °C for Cu-Zn/ZnCl-SiO2) (Figure 5c), which is attributed to the decomposition of Cu-Zn hydroxy-compound, is larger for Cu-Zn/ZnCl-SiO2 than for Cu-Zn/ZnNO3-SiO2 (Figure 5b) or for Cu-Zn/SiO2 (Figure 5a). This is an indication in favor of the formation of a higher amount of the Cu-Zn hydroxy-compound (chloride and nitrate) in Cu-Zn/ZnCl-SiO2. Note that, in the Cu-Zn/SiO2 sample, the Zn loading is lower than in the other two samples (10 instead of 20 wt %). The amount of Zn that can be incorporated in the two mixed hydroxy-compounds must be also considered: According to Jambor et al.,11 the composition in Cu2-xZnx(OH)3Cl may vary between 48.9 and 53.7 wt % for Cu and between 8.7 and 4.2 wt % for Zn, leading to Zn/Cu molar ratios between 0.08 and 0.18. However, according to Mannoorettonil et al.,12 a hydroxychloride compound with a much higher Zn/Cu molar ratio may also form, Zn3.85Cu0.15(OH)8 · ZnCl2 · H2O, from precipitations performed at 75 °C from mixed solutions of CuCl2 + ZnCl2. Regarding Cu2-xZnx(OH)3(NO3), the same authors12 report that the Zn/Cu ratio in hydroxynitrates can continuously vary within a large range up to 1.7 for precipitations from nitrate solution performed at 25 °C and up to 1 for precipitations performed at 75 °C. From these elements of information and the fact that Cu-Zn/ ZnCl-SiO2 also contains hydroxynitrates, it is not possible to conclude whether the Zn/Cu ratio in a mixed compound would be higher in one of the two samples. So, the higher selectivity of the Cu0-ZnII/ZnCl-SiO2 sample can be attributed to the higher amount of Cu-Zn mixed hydroxy-compound, but a higher proportion of Zn in the Cu-Zn hydroxy-compound

Basicity and Cu0-ZnII Interactions present in Cu-Zn/ZnCl-SiO2 than in Cu-Zn/ZnNO3-SiO2 cannot be excluded either. The fact that the selectivities to crotyl alcohol do not change significantly when Cu-Zn/ZnNO3-SiO2 and Cu-Zn/ZnCl-SiO2 are dried at 25 or 90 °C before reduction (Figure 6) in contrast with Cu-Zn/SiO2, confirms that the impregnation step, i.e., the reaction at the solid-liquid interface, is the determinant for the establishment of Cu-Zn interaction in the Cu-Zn/Zn-SiO2 samples. All the results obtained with Cu-Zn/SiO2 (Figure 6B), and with our former studies on Cu/SiO2,1 Zn/SiO2,2 and Cu-Zn/ SiO23 demonstrate that the samples in the reduced state “keep the memory” of their state after drying, i.e., dispersion and nature of the compounds. In the present paper on zinc-modified silica support, we also show that the samples in the reduced state “keep the memory” of their state established during the step of impregnation since the Cu0-ZnII interactions in the reduced samples are related to the close proximity of Cu and Zn induced during impregnation. V. Conclusion The impregnation of silica with zinc nitrate or zinc chloride (10 wt % Zn) followed by calcination at 450 °C, leads to the formation of zinc silicate of hemimorphite-type (Zn4Si2O7(OH)2 · H2O) and isolated grafted zinc species ((tSiO)2ZnCl(O) with (O) ) OH or H2O), respectively. These two zinc species provide basic properties to silica. This is attested by the reaction of conversion of methylbutynol, which also reveals that the hemimorphite leads to more basic material than the grafted zinc species. When copper nitrate is then impregnated on these supports, it is readily transformed into hydroxy-compounds during the impregnation step because of the presence of basic sites, i.e., into hydroxynitrate in the case of hemimorphite and into a mixture of hydroxynitrate and hydroxychloride in the case of grafted Zn. Coimpregnation with a mixture of copper and zinc nitrates favors the formation of mixed Cu-Zn compounds, hydroxynitrate, or hydroxychloride during the impregnation step. The establishment of Cu0-ZnII interaction is attested by the selectivity to crotyl alcohol in the test-reaction of hydrogenation of crotonaldehyde. In this work, we have pointed out the relevance of the nature of the supported mixed

J. Phys. Chem. C, Vol. 114, No. 25, 2010 11147 compounds, which depends on the nature of zinc present on silica support. Zinc silicate of hemimorphite-type favors the formation of Cu-Zn hydroxynitrate, and isolated grafted zinc ((tSiO)2ZnCl(O)) favors the formation of Cu-Zn hydroxychloride and of higher amount of Cu-Zn hydroxynitrate than on zinc silicate. After reduction, not only are more numerous ZnII in interaction with Cu0, but the copper particles are smaller in the latter catalyst. Basicity of the support clearly favors the formation of hydroxy-compound upon copper or copper-zinc impregnation. However, the dispersion and crystallinity of these precursors that directly govern the final catalyst properties are strongly dependent on the nature of the zinc species obtained on modified silica. Acknowledgment. The authors warmly thank Raymonde Touroude (LMSPC, UMR 7515 CNRS, ECPM-Universite´ Louis Pasteur, Strasbourg, France) for lending us her catalytic test of hydrogenation of crotonaldehyde and for technical assistance and scientific discussions. They also thank Dr. Christophe Me´thivier, colleague at the LRS, for the XPS measurements. References and Notes (1) Toupance, T.; Kermarec, M.; Louis, C. J. Phys. Chem. B 2000, 104, 965. (2) Chouillet, C.; Villain, F.; Kermarec, M.; Lauron-Pernot, H.; Louis, C. J. Phys. Chem. B 2003, 107, 3565. (3) Catillon-Mucherie, S.; Ammari, F.; Krafft, J.-M.; Lauron-Pernot, H.; Touroude, R.; Louis, C. J. Phys. Chem. C 2007, 111, 11619. (4) Rodrigues, E. L.; Marchi, A. J.; Apesteguia, C. R.; Bueno, J. M. C. Appl. Catal. A: Gen. 2005, 294, 197. (5) Claus, P. Top. Catal. 1998, 5, 51. (6) Lauron-Pernot, H.; Luck, F.; Popa, J. M. Appl. Catal. 1991, 78, 213. (7) Lauron-Pernot, H. Catal. ReV. 2006, 48, 315. (8) Breuer, K.; Teles, J. H.; Demuth, D.; Hibst, H.; Scha¨fer, A.; Brode, S.; Domgo¨rgen, H. Angew. Chem., Int. Ed. 1999, 38, 1401. (9) Che, M.; Cheng, Z. X.; Louis, C. J. Am. Chem. Soc. 1995, 117, 2008. (10) Zanella, R.; Delannoy, L.; Louis, C. Appl. Catal., A 2005, 291, 62. (11) Jambor, J.; Dutrizac, J.; Roberts, A.; Grice, J.; Szymanski, J. Can. Mineral. 1996, 34, 61. (12) Mannoorettonil, M.; Glibert, J. Bull. Soc. Chim. Belg. 1975, 84, 179.

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