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NaY catalysts
Applied Catalysis A: General 182 (1999) 41±51
Zeolite-supported metals by design: organometallic-based tin-promoted rhodium/NaY catalysts Sandro Recchiaa, Carlo Dossib, Achille Fusib, L. Sordellia, Rinaldo Psaroa,* a
b
CNR Centre ``CSSCMTBSO'', Metallorganica e Analitica ± UniversitaÁ di Milano, Via Venezian 21, 20133, Milano, Italy Dipartimento di Chimica Inorganica, Metallorganica e Analitica ± UniversitaÁ di Milano, Via Venezian 21, 20133, Milano, Italy Received 16 March 1998; received in revised form 28 September 1998; accepted 28 September 1998
Abstract Rhodium±tin bimetallic particles entrapped in NaY cages were used to study the mechanism of tin-promotion in the selective hydrogenation of a,b-unsaturated aldehydes. These model materials were obtained by chemical vapour deposition (CVD) and subsequent H2 reduction of Sn(R)4 (RC2H5; C6H5) onto reduced Rh/NaY samples that were prepared by ionexchange (IE) or by chemical vapour deposition (CVD). In the former case, we have catalysts containing appreciable amounts of proton, while non-acidic metal-in-zeolite samples are obtained with CVD. TPRD studies indicate that the decomposition of tin precursors takes place on the surface of the rhodium particles only if the monolayer capacity is not exceeded. In addition, the mechanism of decomposition is in¯uenced by protons and by the tin precursor used. Carbonyl DRIFT spectra reveal clear evidences of a surface tin-enriched Rh±Sn phase only for proton-free CVD-based samples. In this respect, Sn(C6H5)4 leads to the formation of a higher tin coverage than that obtained from Sn(C2H5)4. In the selective hydrogenation of citral (3,7dimethyl-2,6-octadienal), the presence of protons was highly detrimental leading to the acetal formed by reaction with the solvent (ethanol). With proton-free catalysts, the formation of the saturated aldehyde and of the two unsaturated alcohols is observed. Selectivities could be in¯uenced by both monolayer and multilayer deposits of tin on Rh/NaY. The promotion effect under running catalytic conditions is ascribed to the presence of non-ionic oxidised SnOx phases. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Y zeolite; Rh±Sn encaged particles; Tin promotion mechanism; Citral hydrogenation
1. Introduction The catalytic performances of supported metal catalysts can be substantially changed by tin addition, as demonstrated in hydrocarbon reforming reactions [1], deNOx reactions [2], and hydrogenation of poly-
*Corresponding author. Tel.: +39-2-70600630; fax: +39-22362748; e-mail: [email protected]
unsaturated aldehydes [3]. In this latter respect, tin is probably one of the most powerful and versatile promoting agent, driving the hydrogenation of a,bunsaturated aldehydes towards the selective formation of unsaturated alcohols. The oxidation state of tin, the different nature of active metal±tin phases, and the electronic modi®cations induced by tin addition are still the subject of discussion [4]. The intrinsic dif®culty in acquiring spectroscopic data under reaction conditions is the
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00426-8
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S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
most probable reason for the lack of a satisfactory explanation of the tin promotion mechanism. In the attempt of shading some light on the nature of the active phase well-designed model, catalytic tests have been used and signi®cant results were reported for the selective hydrogenation of acrolein [5,6]. The nature and behaviour of the active sites seem to depend on the M/Sn molar ratio, but considerations on this stoichiometric parameter cannot be separated from the methodology of catalyst preparation and the nature of the tin precursor. The intimate contact between metal and promoter cannot be ensured by the conventional co-impregnation techniques. The limited mobility on the support surface leads to a non-uniform distribution and segregation of the two constituents. Controlled surface reactions have been introduced [7] to overcome all these problems and were successfully applied to the decomposition of tin tetraalkyls deposited on supported metal surfaces [8±12]. In our approach, the zeolites are devised as a nanoscale reactor for the design of tin-promoted bimetallic particles with well-tuned catalytic properties. Zeolites are in fact known for a variety of de®ned cage/channel structures and offer signi®cant advantages over conventional inorganic oxides, either for the stabilisation of small metallic particles induced by geometrical constraints [13] or for the ®ne control of acid properties by exchange of the balancing cations against protons [14]. The promotion effects of tin were studied here by analysing the preparation and characterisation of encaged rhodium±tin particles supported on Y zeolite. The use of rhodium as the metal and Y zeolite as the support was prompted by (i) the large body of spectroscopic and thermochemical characterisation data available for Rh/Y catalysts, and (ii) the suitability of rhodium±tin particles supported on amorphous materials towards the selective catalytic hydrogenation of a,b-unsaturated aldehydes [15±17]. The utilisation of Y zeolites in this ®eld of catalysis has been previously reported on cinnamaldehyde and methyl crotonaldehyde hydrogenation, using supported metals like Pt, Ru and Rh [18±20]. With cinnamaldehyde, the high yields to unsaturated alcohols were ascribed to a shape selectivity effect: the size of zeolite pores does not permit the adsorption of the sterically hindered C=C bond, consequently the adsorption of the carbonyl group at the end of the
unsaturated aldehyde is favoured. Such an effect was not observed with the less bulky methyl-crotonaldehyde: in this case silica-supported catalysts are always more selective than the zeolite supported ones. However, such zeolite-entrapped metallic particles were obtained by the conventional ion exchange technique: consequently, protonic acidity was unavoidably generated upon hydrogen reduction, leading to the formation of a bifunctional catalyst [21]. The use of neutral organometallic complexes as metal precursors would instead allow the preparation of non-acidic metal-in-zeolite catalysts. The organometallic molecule can be introduced selectively into zeolite cages by chemical vapour deposition (CVD), without altering the distribution of intrazeolitic cations. With this method, metal particles are then simply formed by thermal removal of the volatile ligands under reducing conditions, leading to the formation of proton-free encaged metallic particles [22]. We have thus compared the IE and CVD techniques in the preparation of rhodium particles encaged in NaY zeolites, whereas the subsequent deposition of tin was always accomplished by CVD of selected organometallic precursors. The mono- and bimetallic systems were characterised by a multitechnique analytical approach including spectroscopic, thermochemical and conventional H2 chemisorption methodologies. Finally, for the catalytic tests, citral has been used as a model substrate instead of the more bulky cinnamaldehyde with the a priori purpose of avoiding any shape selectivity effect, which could have made dif®cult a correct evaluation of tin promotion effects. 2. Experimental 2.1. Catalysts preparation The starting zeolite was NaY (Aldrich Linde LZY52), containing 56 Na ions per unit cell. Rh-containing NaY samples were prepared by two different methods. One is ion-exchange of [Rh(NH3)5Cl]2 and the samples are denoted as Rh/ NaY(IE). The activation procedure is brie¯y summarised as follows and the full details are reported in literature [23]. After ion exchange, samples were air
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
dried at room temperature for 12 h. Typically 100± 150 mg of dried catalyst were calcined in O2 (50 ml/ min) at 5008C and subsequently reduced in H2 (50 ml/ min) at the same temperature. Alternatively, a chemical vapour deposition of [Rh(CO)2acac] (acacacetylacetonate) has been used, and the samples are denoted as Rh/NaY(CVD). The NaY zeolite has been pretreated in ¯owing Ar at 4508C prior to the deposition of the organometallic precursor. After deposition, the rhodium metallic particles were obtained by thermal decomposition of the entrapped precursor in ¯owing H2 from room temperature to 5008C with a heating rate of 108C/min [23]. The deposition of tin on rhodium metallic particles [Rh/NaY(IE) or Rh/NaY(CVD)] was accomplished by chemical vapour deposition of Sn(C2H5)4 or Sn(C6H5)4 in ¯owing argon at 558C and 2358C, respectively. The resulting material was decomposed in ¯owing hydrogen from room temperature to 5008C with a heating rate of 58C/min. The rhodium loading determined by AAS after chemical dissolution [24] was 10.05 wt% in all the samples. The Sn content, determined by ICPOES, was instead not constant for all samples and it will be speci®ed in the text as Sn/Rh molar ratio. 2.2. Temperature programmed studies The kinetics of thermal decomposition of tin precursors on Rh/NaY samples was studied by temperature programmed reductive decomposition (TPRD) in ¯owing H2 (8%)/He mixture from room temperature to 5008C with a heating rate of 38C/min. All samples coming from the decomposition studies were calcined at 5008C in ¯owing O2 and a temperature programmed reduction (TPR) analysis was then done in ¯owing H2 (8%)/Ar mixture from room temperature to 5008C with a heating rate of 88C/min. Full details for both techniques have been described previously [25,26]. 2.3. Hydrogen chemisorption The amounts of H2 adsorbed on the catalysts were measured in the pulse mode using a Micromeritics Pulse Chemisorption 2700 apparatus. Chemisorption measurements were carried out at 758C with H2 pulses
43
of 50 ml. The chemisorption temperature was determined after a temperature hydrogen desorption measurement from ÿ508C to 5008C. More detailed information about apparatus and procedures are reported elsewhere [25]. 2.4. Diffuse reflectance infrared spectroscopy (DRIFTS) In situ diffuse re¯ectance infrared spectra were recorded on a FTS-40 Digilab FT-IR spectrophotometer equipped with a Harrick Scienti®c DRA3CO diffuse re¯ectance accessory. A high pressure, high temperature environmental chamber for DRIFT studies (Harrick Scienti®c mod. HVC-DR3) allowed spectra of samples in powder form to be recorded in ¯ow conditions under controlled pressure and temperature. After TPRD investigation, the samples were transferred in air into the DRIFT cell and then reduced for 1 h in H2 ¯ow at 5008C (heating rate 108C/min) and cooled to room temperature in N2 ¯ow. Carbonylation was done in a pure CO ¯ow (6 ml/ min) at atmospheric pressure and room temperature. For every spectrum, 100 scans were collected at 4 cmÿ1 resolution. The interferograms were converted by means of the Kubelka±Munk transformation and plotted against wavenumbers. 2.5. Catalytic studies The hydrogenation of citral (mixture of cis- and trans-isomers, Aldrich C8,300-7, purity 95%) has been carried out in a 100 ml glass batch reactor thermostatted at 358C under H2 atmosphere. These very mild conditions were chosen to avoid diffusional limitations [27]. Typically, 20 ml of a 1:200 (v/v) citral:ethanol solution was loaded into the reactor; the amount of catalyst was such to have a substrate:rhodium molar ratio of about 200. The reaction was followed by analysing a suf®cient number of microsamples. Quantitative analysis was performed with a Helwett-Packard MSD 5971 GC± MS apparatus equipped with a DB-225 J&W column (0.23 mm i.d.30 m length). Catalytic conversions are calculated on citral consumption. Yields to cis- and trans-3,7-dimethyl-2,6octadien-1-ol (unsaturated alcohols ± UOLs) are
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S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
reported as (moles of UOLs)/(total moles of citral reacted)100. Yields to 3,7-dimethyl-6-octenal (saturated aldehyde ± SAL) and to acetal are reported similarly. To avoid the formation of the dietoxyacetal, some catalytic tests were carried out in n-heptane and nhexane. However, with these solvents, diffusion of reactants and products inside the zeolitic channels becomes the real rate determining step. 3. Results and discussion Cage-entrapped rhodium particles with or without intrazeolitic protons were obtained by ion-exchange (Rh/NaY(IE)) or by chemical vapour deposition (Rh/ NaY(CVD)), respectively. On these starting materials, organometallic tin precursors were then deposited by CVD. 3.1. Thermochemical characterisation Temperature programmed reductive decomposition (TPRD) analyses were carried out to study the decomposition of tin precursors as a function of the presence of protons and of the nature of tin complexes. In Fig. 1, the TPRD pro®les of Sn(C2H5)4 deposited on Rh/NaY(CVD) and on Rh/NaY(IE) are reported. On Rh/NaY(CVD) (Fig. 1(b)), the most abundant decomposition product is ethane, formed by b-elimination in H2 atmosphere: Rh0x -Sn C2 H5 4 =NaY2H2 ! Rh0x -Sn=NaY 4C2 H6 (1) The presence of two overlapped evolution peaks with maxima at 1608C and 2008C suggests a 2-step mechanism for the stoichiometric reaction (1). The intermediate formation of Sn(C2H5)x (x2) fragments supported onto the rhodium metal particle partially follows the ®ndings of Didillon et al. [16] for tetrabutyl tin on Rh/SiO2. The authors were in fact able to obtain stable Rh±Sn(alkyl)2 surface organometallic species by stopping the reduction process around 1308C: in our situation, the extensive overlap of the two decomposition steps does not allow the isolation of partially reduced Sn(C2H5)2 species, which rapidly decomposes just after formation.
Fig. 1. TPRD profiles of Sn(C2H5)4 deposited on: (A) Rh/ NaY(IE), Sn/Rh1.0; (B) Rh/NaY(CVD), Sn/Rh1.0; and (C) Rh/NaY(CVD), Sn/Rh2.0. Heavy line: m/z28, thin line: m/z15.
Methane evolution at 2808C indicates that a minor part of ethyl fragments are removed following a hydrogenolysis pathway assisted by the intrazeolitic rhodium metallic particles. A similar behaviour was already observed in the decomposition of Sn(Bu)4 on Rh/SiO2 [28], where ethane was evolved as a minor, high-temperature hydrogenolysis product.
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
In the TPRD pro®le of Rh/NaY(IE) (Fig. 1(a)), where intrazeolitic protons were previously formed upon Rh(III) ion reduction, the methane evolution peak at about 2508C is far more pronounced than that obtained for the proton-free system. Moreover, on Rh/ NaY(IE), the evolution of ethane is now observed as a single and narrower peak at about 1808C. The intermediate formation of Sn(C2H5)x fragments, which are subsequently decomposed following a hydrogenolysis pathway, is thus strongly suggested. The increased methane evolution is now to be attributed to the hydrogenolytic removal of carbon residues by carbenium ion reactions of ethyl fragments in the presence of protons [29]. An increase in the amount of tin precursor (Sn/Rh molar ratio of about 2) causes the appearance in the TPRD pro®le (Fig. 1(c)) of a new and narrow peak of ethane at about 908C. Such evolution is consistent with the thermal decomposition of some Sn(C2H5)4 without any direct interaction with Rh particles. Subsequent experiments have shown that the narrow peak at 908C appears only when the Sn/Rh molar ratio exceeds 1.2±1.3. Since it is known that increasing the amount of tin precursor leads to a gradual coverage of the rhodium phase [30,31], our data strongly suggest that a complete saturation of the rhodium sites is reached at high Sn/Rh values. In other words, Sn(C2H5)4 is ®rst chemisorbed onto Rh particles and after that, a multilayer adsorption is taking place. This conclusion is also enforced by the observation that we were not able to deposit the tin precursor in the absence of the preformed intrazeolitic rhodium particles. The use of the Sn(C6H5)4 precursor leads to completely different results, owing to its high thermal stability requiring a sublimation temperature about 1808C higher than that of Sn(C2H5)4. The decompositions of this tin precursor deposited on Rh/NaY(IE) and on Rh/NaY(CVD) are shown in Fig. 2. In both cases, the only relevant peak concerns methane evolution around 3008C: no peaks at m/z77, diagnostic of benzene desorption, were observed. Noteworthy, the close similarity of the two pro®les of Fig. 2 clearly indicates that the reactivity is independent from the presence of protons, and hence that hydrogenolysis is the only decomposition process taking place. To our opinion, the high thermal stability of Sn(C6H5)4 could explain why a reductive elimina-
45
Fig. 2. TPRD profiles of Sn(C6H5)4 deposited on: (A) Rh/ NaY(IE), Sn/Rh1.0; and (B) Rh/NaY(CVD), Sn/Rh1.0. Heavy line: m/z77, thin line: m/z15.
tion pathway similar to reaction (1) is non-operative in this case: the hydrogenolysis pathway is in fact thermodynamically strongly favoured at high temperatures. From the TPRD studies, two observations can be drawn: 1. the decomposition of tin precursors takes place on the surface of the small rhodium particles only if the monolayer capacity is not overcome; and 2. the mechanism of decomposition is influenced by the presence of protons and by the tin precursors used. The TPR technique has been used in order to get information about the nature of the entrapped bimetallic particles and hence on the extent of the rhodium± tin interactions. The TPR pro®les of both Rh/NaY(CVD) and Rh/ NaY(IE) monometallic catalysts are well described in
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S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
the literature [32,33]. For the CVD sample, a simple reduction peak at 608C, assigned to the reduction of Rh2O3 to Rh0, is observed. In the case of IE sample, two peaks at 608C and 2708C are usually found. The low temperature peak is attributed to the reduction of Rh2O3 and/or RhO (being formed by reaction of Rh0 with protons), while the other one to the more thermodynamically stable Rh3 cations located in the super and/or in the sodalite cages of the NaY, that can only be produced in the presence of H: Rh2 O3 6H ! 2Rh3 3H2 O
(2)
Rh0 3H ! Rh3 3=2 H2
(3)
The reduction pro®le of a Sn/NaY prepared by ion exchange from SnCl2 (owing to the impossibility of preparing Sn/NaY samples by CVD) is shown in Fig. 3(a): a broad peak that starts at about 3508C but is still not complete at 5008C is observed. More-
Fig. 3. TPR profiles after calcination at 5008C: (A) Sn/NaY(IE); (B) Sn±Rh/NaY(IE) obtained from Sn(C6H5)4 (Sn/Rh0.38); (C) Sn±Rh/NaY(IE) obtained from Sn(C2H5)4 (Sn/Rh0.57); and (D) Sn±Rh/NaY(IE) obtained from Sn(C6H5)4 (Sn/Rh2.0).
over, the total amount of hydrogen consumed up to 5008C can only account for a partial reduction of tin. A similar behaviour was observed for Mn/NaY samples prepared by ion exchange, where no reduction peaks were observed at all: in that situation, the segregation of the manganese cations into the small sodalite cages was strongly suggested [33,23]. The high ®eld stabilisation exerted by sodalite cages onto bivalent cations could then be invoked also in this case to explain the strong tendency of tin to remain in the oxidised state. When tin is deposited by CVD onto pre-reduced Rh/NaY(IE), the TPR pro®les signi®cantly change. As it can be seen in Fig. 3(b) and (c), the reduction maxima are located at exactly the same position independently from the nature of the tin precursor: two distinct low temperature reductions are observed at 608C and 808C, with two other broader peaks at 2308C and 2708C. In both the cases, the total amount of hydrogen consumed accounts for the complete reduction of SnIV to Sn0 and of RhIII to Rh0. It is noteworthy that the amount of H2 consumed below 1508C is much higher than that required for the total rhodium reduction, meaning that a partial reduction of tin is also involved in this region. Moreover, the peak at 808C shows a marked increase passing from a Sn/Rh value of 0.38 to 0.57 and ®nally to 2.0 (Fig. 3(b)±(d)). This peak could then be unambiguously assigned to the reduction of a SnOx phase. As in the monometallic rhodium catalyst, the peak at 608C is instead attributed to the reduction of Rh2O3 and/or RhO. Similarly (again compare Fig. 3(b)±(d)), the two high temperature peaks at 2308C and 2708C are, respectively, assigned to the reduction of Snn and Rh3 ionic species, being formed by the reaction of the oxidised phases with intrazeolitic protons. The enhanced reducibility of Sn in the presence of Rh particles, as previously observed for many bimetallic catalysts in zeolite [34], is a direct indication of the intimate contact of the two metals in the bimetallic catalyst. Another interesting feature of the high tin loaded TPR pro®le (Fig. 3(d)) is the relevant hydrogen consumption above 3508C. This last feature closely resembles the behaviour of Sn/NaY, which indicates that at least a part of tin is no more in contact with the
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Table 1 Hydrogen chemisorption data Catalyst
Precursor
Sn/Rh
H/Rh
Sn/Y(IE) Rh/NaY(CVD) Rh/NaY(IE) Sn±Rh/Y(IE) Sn±Rh/Y(IE) Sn±Rh/Y(CVD) Sn±Rh/Y(CVD)
SnCl2 Rh(acac)(CO)2 [Rh(NH3)5Cl]2 Sn(C6H5)4 Sn(C2H5)4 Sn(C6H5)4 Sn(C2H5)4
± ± ± 0.38 0.57 0.49 0.51
0 0.92 1.04 0.67 0.56 0.40 0.70
3.2. Chemisorption measurements
Fig. 4. TPR profiles after calcination at 5008C: (A) Sn±Rh/ NaY(CVD) obtained from Sn(C6H5)4 (Sn/Rh0.49); and (B) Sn± Rh/NaY(CVD) obtained from Sn(C2H5)4 (Sn/Rh0.51).
rhodium phase. These results are in agreement with the TPRD data observed when the Sn/Rh molar ratio of about 1.2±1.3 is overcome (compare Fig. 1(b) and (c)). The TPR pro®les of the proton-free Sn±Rh/ NaY(CVD) samples are reported in Fig. 4. As in the case of the related IE systems, the amount of hydrogen consumed accounts for the complete reduction of SnIV to Sn0 and of RhIII to Rh0, both for the Sn(C2H5)4 and for the Sn(C6H5)4 based samples. Moreover, recalling that in the absence of intrazeolitic protons, no ionic Rh3 or Snn species can be present, all TPR peaks have to be attributed to neutral oxidised species only. However, the TPR pro®les are now much more complex than those of the Sn±Rh/NaY(IE) samples, and do not allow a clear identi®cation of the reduction processes being involved. Experiments carried out changing the Sn/Rh molar ratio have shown a great variability both of peaks height and of peaks position, indicating that in the absence of protons, the nature of the rhodium±tin phase is also composition-dependent.
The hydrogen chemisorption measurements carried out in the pulse mode on mono- and bimetallic catalysts are reported in Table 1. Both IE and CVD monometallic Rh/NaY catalysts show H/Rh values near unity that, according to a 1:1 H:M stoichiometry [35], indicate a good dispersion level. Moreover, tin does not chemisorb H2, as previously reported [36]. The bimetallic catalysts show chemisorption values lower than their respective Rh/NaY systems. Since it is reasonable to suppose that inside the zeolite cages, the dimensions of the rhodium particles do not increase upon deposition of the tin phase, the lower H/Rh values reported here have to be attributed to a partial tin coverage of the exposed rhodium surface. The CVD-based Sn±Rh/NaY samples show different H/Rh chemisorption values as a function of the tin precursor. The chemisorption data indicate a more pronounced tin coverage for the Sn(C6H5)4 derived catalysts than for the Sn(C2H5)4 one. Our data would suggest that the utilisation of Sn(C6H5)4 leads to the formation of bimetallic particles with a higher tin coverage than that obtained from Sn(C2H5)4. Additional support to this suggestion is provided by our IR spectra. The two bimetallic catalysts derived from Rh/ NaY(IE) samples show instead similar dispersion, with the higher H/Rh value observed for the sample with the lowest Sn/Rh ratio. According to the TPR measurements, these chemisorption data con®rm that, in the presence of protons, the nature of the entrapped rhodium±tin particles does not depend upon the tin precursor.
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Fig. 5. DRIFT spectra of Sn±Rh/NaY(IE) obtained from Sn(C6H5)4: (A) after 180 min in flowing CO at room temperature; and (B) from (A) after 30 min of purging in Ar flow at room temperature.
3.3. DRIFT spectra of adsorbed CO Diffuse re¯ectance infrared FT (DRIFT) spectra of the Sn(C6H5)4-derived Sn±Rh/NaY(IE) sample (Fig. 5) after carbonylation are substantially similar to those obtained with monometallic Rh/NaY(IE) [37]. In that case, the bands above 2000 cmÿ1 were attributed to the presence of two Rh(I) gem-dicarbonyl species [32,38], while the partial formation of Rh4(CO)12, with its doubly bridged CO ligands, is responsible for the 1830 cmÿ1 absorption [39]. This latter species is stable only in CO atmosphere and tends to reorganise to the more stable Rh6(CO)16 when
CO is purged away: during the interconversion a rapid decrease of the 1834 cmÿ1 is observed, which parallels the raise of the 1763 cmÿ1 band, due to triply bridged CO ligands [23]. As it can be seen in Fig. 5, similar changes are observed for Sn±Rh/NaY(IE), meaning that this catalyst closely resembles the behaviour of the monometallic Rh/NaY(IE) system. Since from the TPR analysis, we have unambiguously concluded that the easy reducibility of tin has to be ascribed to the intimate contact with the Rh particles, it seems that such contact cannot be observed by IR analysis. However, we have to consider that in the presence of CO and protons, an oxidative disruption process of Rh particles occurs [38], causing the formation of Rh CO 2 and the subsequent removal of Rh±Sn interactions. From this point of view CO does not properly act as a probe molecule, since in the presence of protons it is able to modify the nature of the entrapped particles. Clear evidences of bimetallic Rh±Sn interactions are observed only with CVD catalysts, as the oxidative disruption process cannot be active not having protons. Signi®cant differences are in fact observed when the DRIFT spectra of Fig. 6(c) and (d) (relative to CVD-based catalysts obtained using Sn(C6H5)4 and Sn(C2H5)4, respectively) are compared with those of the monometallic Rh/Y(CVD) catalyst (Fig. 6(a) and (b)). Bands due to terminal CO ligands between 2100 and 2000 cmÿ1 differ substantially from the ones of the Rh/NaY(CVD) sample. On the contrary, the band at 1830 cmÿ1 is observed even for both bimetallic samples. However, differently from the monometallic sample [23], this band does not disappear when CO is removed by ¯owing inert gas. On this basis, we would reject the attribution of the 1830 cmÿ1 band to the formation of Rh4(CO)12, since its transformation into the hexanuclear cluster is a facile process, being always observed both in homogeneous [40] and in heterogeneous phase [41]. The double bridging coordination of CO may result from a preferential attachment of tin onto the three-fold hollow sites of the pre-formed rhodium particles, preventing CO to coordinate in the triply bridging form. The preferential location of tin on the more coordinatively unsaturated rhodium sites was in fact previously reported by Coq et al. [31]. It is noteworthy that the total amount of adsorbed CO decreases passing from Sn(C2H5)4 to Sn(C6H5)4-
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dimethyl-2,6-octadienal). The catalytic data are summarised in Table 2. With the monometallic Rh/NaY(IE) catalyst, we observe both the formation of the diethyl acetal, and the hydrogenation of the conjugated C=C double bond to give the saturated aldehyde citronellal (SAL). Instead, the proton-free Rh/NaY(CVD) catalyst does not generate any signi®cant amount of acetal, with the saturated aldehyde being the only reaction product. Hence, the presence of intrazeolitic protons is highly detrimental, effectively catalysing the acetal formation. On the bimetallic Sn±Rh/NaY(IE) catalysts, the yield of the acetal is much higher than that of the monometallic Rh/NaY(IE) system, with citronellal being only a minor product. The additional presence of Sn2 Lewis acid sites, formed through one of these pathways, Sn0 2H ! Sn2 H2 Sn0 H2 O ! SnO H2 # SnO 2H ! Sn2 H2 O
Fig. 6. DRIFT spectra of CVD-based mono- and bimetallic catalysts: (A) Rh/NaY(CVD) after 180 min in flowing CO at room temperature; (B) from (A) after 30 min of purging in Ar flow at room temperature; (C) Sn±Rh/NaY(CVD) obtained from Sn(C6H5)4 after complete carbonylation and subsequent purging in Ar flow; and (D) Sn±Rh/NaY(CVD) obtained from Sn(C2H5)4 after complete carbonylation and subsequent purging in Ar flow.
derived catalysts (compare Fig. 6(a) and (b)). In agreement with the conclusions gained on the basis of thermochemical and chemisorption data, we may con®rm that the latter sample shows a more pronounced tin coverage. 3.4. Catalytic activity The possibility of modelling rhodium±tin interactions has been monitored by testing our bimetallic catalysts in the selective hydrogenation of citral (3,7-
(4) (5)
easily accounts for the catalytic condensation to acetal and was also already reported by Galvagno et al. [42] on Ru±Sn catalysts for the selective hydrogenation of cinnamaldehyde. The utilisation of a reacting solvent like ethanol plays thus a key role allowing to detect the presence of Brùnsted and Lewis acid sites. In fact, as discussed above for the Rh/NaY(CVD) system, the formation of acetal is completely inhibited also with the proton- and Lewis acid-free bimetallic Sn±Rh/ NaY(CVD) catalysts. Moreover, a competition towards the attack of conjugated C=C and C=O is instead observed with the bimetallic systems. In this case, the major reaction products are the saturated aldehyde and the two unsaturated alcohols, cis- and trans-3,7-dimethyl-2,6-octadien-1-ol (UOLs), deriving from cis- and trans-citral. As has been reported for tin promoted catalysts [4], the hydrogenation of C=O requires an activation time: during this period, the reaction of the conjugated C=C is quite fast and decreases in favour of C=O hydrogenation. The active surface must hence experience a partial modi®cation to become active towards the hydrogenation of C=O. Since in the starting catalyst both metals are in the zero-valent state, we have to conclude that, as
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Table 2 Catalytic data for citral hydrogenation Catalyst Rh/NaY(IE)a Rh/NaY(CVD)a Sn±Rh/NaY(IE)a Sn±Rh/NaY(IE)a Sn±Rh/NaY(CVD)b Sn±Rh/NaY(CVD)b a b
Tin precursor
Conversion (%)
Sn(C6H5)4 Sn(C2H5)4 Sn(C6H5)4 Sn(C2H5)4
60 63 63 62 45 47
Product distribution (%) Acetal
SAL
UOLs
56 ± 88 86 ± ±
43 98 12 14 92 61
± ± ± ± 8 39
After 1400 min. After 2500 min.
reported for Rh±Sn/SiO2 catalysts [15], a Rh0±Sn0 phase is reasonably not active towards the C=O hydrogenation. A partial oxidation of the tin phase is strongly suggested to occur during the induction period, as reported by Marinelli and Ponec [5]. However, we have to exclude the presence of ionic Sn2 because (i) no acetal is formed with these catalysts, and (ii) the charge balance of the zeolite does not allow the formation of any isolated ion. The presence of a neutral tin oxidised phase (SnOx) is then the most favoured as the actual promoter for C=O bond activation. 4. Conclusions There are two strategies to design the rhodium±tin interactions, namely (i) the introduction of surface protonic acidity, and (ii) a change in the reactivity of the volatile tin precursor. The presence of intrazeolite protonic acidity is shown to promote Sn2 formation either by reoxidation of the zerovalent Sn phase or by acid±base reaction with SnO (or SnO2) formed by reaction with water (reactions 4 and 5). The effect of the presence of protons is even more dramatic in CO-containing atmosphere, since the oxidative disruption of Rh particles to Rh CO 2 , totally destroys Rh±Sn interactions. It is thus possible to progressively shift from a true Rh0±Sn0 phase to Rh±Sn (2) as a simple function of the proton content of the zeolite structure. In this respect, the use of chemical vapour deposition might be considered far superior to ion-exchange, since the H/metal ratio is
no more limited by the stoichiometry of ion reduction, but it can be freely varied by simply substituting the alkaline (Na or K) compensating cations with protons in the starting zeolite [14]. In proton-free environments, the intrinsic thermal stability of the tin alkyl precursor may also in¯uence the topology of the bimetallic Rh±Sn particles. At low temperatures (below 2008C), decomposition of the tin alkyl precursor tends to follow a two-step SnR4 ! SnRx ! Sn0 mechanism onto speci®c sites, probably three-fold hollow sites, of the entrapped Rh clusters, leading to the formation of bimetallic Rh0±Sn0 particles. The high temperature decomposition of a stable tin precursor such as Sn(C6H5)4 occurs instead preferentially by the hydrogenolytic rupture of Sn±R bonds, and results in surface-tin-enriched, cherry-type particles. As we have observed with chemisorption and DRIFT analysis, the tin surface coverage is in fact much more pronounced when Sn(C6H5)4 instead of Sn(C2H5)4 is used. Differences in the decomposition pathways are thought to be responsible for these coverage dissimilarities. With Sn(C2H5)4, the multi-step decomposition mechanism necessarily implies that the tin precursor is partially decomposed on the just formed Sn0 species, even if the monolayer capacity of Rh0 particles is still not exhausted. This situation, represented by I in the scheme reported below, is similar to the model and its supporting arguments proposed by Didillon et al. [28] for tin promoted Rh/SiO2 systems. With Sn(C6H5)4, the evidence of a concerted decomposition process implies that the major part of tin precursor molecules reacts on super®cial rhodium atoms, leading to a fairly monolayered bimetallic particle, represented by model II:
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
These models may account for the different rhodium coverage observed in the two cases. Moreover, since activity towards C=O hydrogenation can be related to the concentration of accessible rhodium± tin ensembles, the type I model may also account for the higher selectivity to UOLs observed for the promoted catalyst derived from Sn(C2H5)4. The catalytic tests afforded important information on the nature of the tin promotion effect. This is due to the presence of an oxidised tin non-ionic phase, formed under catalytic conditions, and operating through a Lewis acid±base interaction between the carbonyl group and the exposed tin atom at the edge of the oxide moieties. Acknowledgements We gratefully acknowledge ®nancial support from National Research Council, and Ministry of University and Scienti®c and Technological Research. References [1] J.L. Margitfalvi, M. HegeduÈs, E. TaÂlas, J. Mol. Catal. 51 (1989) 279. [2] K. Tomishige, K. Asakura, Y. Iwasawa, J. Catal. 149 (1994) 70. [3] R.L. Augustine, L. Meng, in: R.E. Malz Jr. (Ed.), Catalysis of Organic Reactions, Marcel Dekker, New York, 1996, p. 15. [4] V. Ponec, Appl. Catal. A 149 (1997) 27. [5] T.B.L.W. Marinelli, V. Ponec, J. Catal. 156 (1995) 51. [6] T.B.L.W. Marinelli, J.H. Vleeming, V. Ponec, in: Proceedings of the 10th International Congress on Catalysis, Vol. B, Budapest, 1992, p. 1211. [7] Yu.I. Yermakov, B.N. Kuznetsov, J. Mol. Catal. 9 (1980) 13. [8] J.L. Margitfalvi, E. TaÂlas, S. GoÈboÈloÈs, Catal. Today 6 (1989) 73. [9] J.P. Candy, A. El Mansour, O.A. Ferretti, G. Mabilon, J.P. Bournonville, J.M. Basset, G. Martino, J. Catal. 112 (1988) 201. [10] J.P. Candy, O.A. Ferretti, G. Mabilon, J.P. Bournonville, A. El Mansour, J.M. Basset, G. Martino, J. Catal. 112 (1988) 210. [11] M. Agnelli, P. Louessard, A. El Mansour, J.P. Candy, J.P. Bournonville, J.M. Basset, Catal. Today 6 (1989) 63.
51
[12] O.A. Ferretti, C. Lucas, J.P. Candy, J.M. Basset, B. Didillon, F. Le Peltier, J. Mol. Catal. 103 (1995) 125. [13] M. Bellatreccia, R. Zanoni, C. Dossi, R. Psaro, S. Recchia, G. Vlaic, J. Chem. Soc., Faraday Trans. 91(13) (1995) 2045. [14] C. Dossi, R. Psaro, R. Ugo, Z.C. Zhang, W.M.H. Sachtler, J. Catal. 149 (1994) 92. [15] B. Didillon, A. El Mansour, J.P. Candy, J.P. Bournoville, J.M. Basset, Stud. Surf. Sci. Catal. 59 (1991) 137. [16] B. Didillon, J.P. Candy, A. El Mansour, C. Houtmann, J.M. Basset, J. Mol. Catal. 74 (1992) 43. [17] B. Didillon, J.P. Candy, F. Le Peltier, O.A. Ferretti, J.M. Basset, Stud. Surf. Sci. Catal. 78 (1993) 147. [18] D.G. Blackmond, R. Oukaci, B. Blanc, P. Gallezot, J. Catal. 131 (1991) 401. [19] P. Gallezot, A. Giroir-Fendler, D. Richard, in: W.E. Pascoe (Ed.), Catalysis of Organic Reactions, Marcel Dekker, New York, 1992, p. 1. [20] D.G. Blackmond, A. Waghray, R. Oukaci, B. Blanc, P. Gallezot, Stud. Surf. Sci. Catal. 59 (1991) 145. [21] W.M.H. Sachtler, Z.C. Zhang, Adv. Catal. 39 (1993) 129. [22] C. Dossi, R. Psaro, A. Bartsch, E. Brivio, A. Galasco, P. Losi, Catal. Today 17 (1993) 527. [23] T. Beutel, H. KnoÈzinger, H. TrevinÄo, Z.C. Zhang, W.M.H. Sachtler, C. Dossi, R. Psaro, R. Ugo, J. Chem. Soc., Faraday Trans. 90 (1994) 1335. [24] C. Dossi, P. Losi, S. Calmotti, Ann. Chim. 83 (1993) 233. [25] C. Dossi, A. Fusi, O. Russo, Analysis Europa, April 1995, p. 17. [26] C. Dossi, A. Fusi, R. Psaro, Thermochim. Acta 236 (1994) 165. [27] A. Waghray, R. Oukaci, D.G. Blackmond, Stud. Surf. Sci. Catal. 75 (1993) 2479. [28] B. Didillon, C. Houtmann, T. Shay, J.P. Candy, J.M. Basset, J. Am. Chem. Soc. 115 (1993) 9380. [29] P.J.M. Rek, A.J. den Hartog, V. Ponec, Appl. Catal. 46 (1989) 213. [30] J.P. Candy, B. Didillon, E.L. Smith, T.B. Shay, J.M. Basset, J. Mol. Catal. 86 (1994) 179. [31] B. Coq, A. Goursot, T. Tazi, F. FigueÂras, D.R. Salahub, J. Am. Chem. Soc. 113 (1991) 1485. [32] D.C. Tomczak, V.L. Zholobenko, H. TrevinÄo, G.-D. Lei, W.M.H. Sachtler, Stud. Surf. Sci. Catal. 84 (1994) 893. [33] C. Dossi, A. Fusi, S. Recchia, R. Psaro, Mater. Eng. 5 (1994) 249. [34] J.S. Feeley, W.M.H. Sachtler, J. Catal. 131 (1991) 573. [35] H. Hamada, R. Funaky, Y. Kuwahara, Y. Kintaichi, K. Wakabayashi, T. Ito, Appl. Catal. 30 (1987) 177. [36] H. Veerbeek, W.M.H. Sachtler, J. Catal. 42 (1976) 257. [37] M. Primet, J. Chem. Soc., Faraday Trans. I 74 (1978) 2570. [38] G. Bergeret, P. Gallezot, P. GeÂlin, Y. Ben Taarit, F. Lefebvre, C. Naccache, R.D. Shannon, J. Catal. 104 (1987) 279. [39] L.-F. Rao, A. Fukuoka, N. Kosugi, H. Kuroda, M. Ichikawa, J. Phys. Chem. 94 (1990) 5317. [40] P. Chini, S. Martinengo, Inorg. Chim. Acta 3 (1969) 315. [41] A. Theolier, A.K. Smith, M. Leconte, J.M. Basset, G.M. Zanderighi, R. Psaro, R. Ugo, J. Organometal. Chem. 191 (1980) 415. [42] S. Galvagno, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 8 (1991) 9.
Zeolite-supported metals by design: organometallic-based tin-promoted rhodium/NaY catalysts Sandro Recchiaa, Carlo Dossib, Achille Fusib, L. Sordellia, Rinaldo Psaroa,* a
b
CNR Centre ``CSSCMTBSO'', Metallorganica e Analitica ± UniversitaÁ di Milano, Via Venezian 21, 20133, Milano, Italy Dipartimento di Chimica Inorganica, Metallorganica e Analitica ± UniversitaÁ di Milano, Via Venezian 21, 20133, Milano, Italy Received 16 March 1998; received in revised form 28 September 1998; accepted 28 September 1998
Abstract Rhodium±tin bimetallic particles entrapped in NaY cages were used to study the mechanism of tin-promotion in the selective hydrogenation of a,b-unsaturated aldehydes. These model materials were obtained by chemical vapour deposition (CVD) and subsequent H2 reduction of Sn(R)4 (RC2H5; C6H5) onto reduced Rh/NaY samples that were prepared by ionexchange (IE) or by chemical vapour deposition (CVD). In the former case, we have catalysts containing appreciable amounts of proton, while non-acidic metal-in-zeolite samples are obtained with CVD. TPRD studies indicate that the decomposition of tin precursors takes place on the surface of the rhodium particles only if the monolayer capacity is not exceeded. In addition, the mechanism of decomposition is in¯uenced by protons and by the tin precursor used. Carbonyl DRIFT spectra reveal clear evidences of a surface tin-enriched Rh±Sn phase only for proton-free CVD-based samples. In this respect, Sn(C6H5)4 leads to the formation of a higher tin coverage than that obtained from Sn(C2H5)4. In the selective hydrogenation of citral (3,7dimethyl-2,6-octadienal), the presence of protons was highly detrimental leading to the acetal formed by reaction with the solvent (ethanol). With proton-free catalysts, the formation of the saturated aldehyde and of the two unsaturated alcohols is observed. Selectivities could be in¯uenced by both monolayer and multilayer deposits of tin on Rh/NaY. The promotion effect under running catalytic conditions is ascribed to the presence of non-ionic oxidised SnOx phases. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Chemical vapour deposition; Y zeolite; Rh±Sn encaged particles; Tin promotion mechanism; Citral hydrogenation
1. Introduction The catalytic performances of supported metal catalysts can be substantially changed by tin addition, as demonstrated in hydrocarbon reforming reactions [1], deNOx reactions [2], and hydrogenation of poly-
*Corresponding author. Tel.: +39-2-70600630; fax: +39-22362748; e-mail: [email protected]
unsaturated aldehydes [3]. In this latter respect, tin is probably one of the most powerful and versatile promoting agent, driving the hydrogenation of a,bunsaturated aldehydes towards the selective formation of unsaturated alcohols. The oxidation state of tin, the different nature of active metal±tin phases, and the electronic modi®cations induced by tin addition are still the subject of discussion [4]. The intrinsic dif®culty in acquiring spectroscopic data under reaction conditions is the
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00426-8
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S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
most probable reason for the lack of a satisfactory explanation of the tin promotion mechanism. In the attempt of shading some light on the nature of the active phase well-designed model, catalytic tests have been used and signi®cant results were reported for the selective hydrogenation of acrolein [5,6]. The nature and behaviour of the active sites seem to depend on the M/Sn molar ratio, but considerations on this stoichiometric parameter cannot be separated from the methodology of catalyst preparation and the nature of the tin precursor. The intimate contact between metal and promoter cannot be ensured by the conventional co-impregnation techniques. The limited mobility on the support surface leads to a non-uniform distribution and segregation of the two constituents. Controlled surface reactions have been introduced [7] to overcome all these problems and were successfully applied to the decomposition of tin tetraalkyls deposited on supported metal surfaces [8±12]. In our approach, the zeolites are devised as a nanoscale reactor for the design of tin-promoted bimetallic particles with well-tuned catalytic properties. Zeolites are in fact known for a variety of de®ned cage/channel structures and offer signi®cant advantages over conventional inorganic oxides, either for the stabilisation of small metallic particles induced by geometrical constraints [13] or for the ®ne control of acid properties by exchange of the balancing cations against protons [14]. The promotion effects of tin were studied here by analysing the preparation and characterisation of encaged rhodium±tin particles supported on Y zeolite. The use of rhodium as the metal and Y zeolite as the support was prompted by (i) the large body of spectroscopic and thermochemical characterisation data available for Rh/Y catalysts, and (ii) the suitability of rhodium±tin particles supported on amorphous materials towards the selective catalytic hydrogenation of a,b-unsaturated aldehydes [15±17]. The utilisation of Y zeolites in this ®eld of catalysis has been previously reported on cinnamaldehyde and methyl crotonaldehyde hydrogenation, using supported metals like Pt, Ru and Rh [18±20]. With cinnamaldehyde, the high yields to unsaturated alcohols were ascribed to a shape selectivity effect: the size of zeolite pores does not permit the adsorption of the sterically hindered C=C bond, consequently the adsorption of the carbonyl group at the end of the
unsaturated aldehyde is favoured. Such an effect was not observed with the less bulky methyl-crotonaldehyde: in this case silica-supported catalysts are always more selective than the zeolite supported ones. However, such zeolite-entrapped metallic particles were obtained by the conventional ion exchange technique: consequently, protonic acidity was unavoidably generated upon hydrogen reduction, leading to the formation of a bifunctional catalyst [21]. The use of neutral organometallic complexes as metal precursors would instead allow the preparation of non-acidic metal-in-zeolite catalysts. The organometallic molecule can be introduced selectively into zeolite cages by chemical vapour deposition (CVD), without altering the distribution of intrazeolitic cations. With this method, metal particles are then simply formed by thermal removal of the volatile ligands under reducing conditions, leading to the formation of proton-free encaged metallic particles [22]. We have thus compared the IE and CVD techniques in the preparation of rhodium particles encaged in NaY zeolites, whereas the subsequent deposition of tin was always accomplished by CVD of selected organometallic precursors. The mono- and bimetallic systems were characterised by a multitechnique analytical approach including spectroscopic, thermochemical and conventional H2 chemisorption methodologies. Finally, for the catalytic tests, citral has been used as a model substrate instead of the more bulky cinnamaldehyde with the a priori purpose of avoiding any shape selectivity effect, which could have made dif®cult a correct evaluation of tin promotion effects. 2. Experimental 2.1. Catalysts preparation The starting zeolite was NaY (Aldrich Linde LZY52), containing 56 Na ions per unit cell. Rh-containing NaY samples were prepared by two different methods. One is ion-exchange of [Rh(NH3)5Cl]2 and the samples are denoted as Rh/ NaY(IE). The activation procedure is brie¯y summarised as follows and the full details are reported in literature [23]. After ion exchange, samples were air
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
dried at room temperature for 12 h. Typically 100± 150 mg of dried catalyst were calcined in O2 (50 ml/ min) at 5008C and subsequently reduced in H2 (50 ml/ min) at the same temperature. Alternatively, a chemical vapour deposition of [Rh(CO)2acac] (acacacetylacetonate) has been used, and the samples are denoted as Rh/NaY(CVD). The NaY zeolite has been pretreated in ¯owing Ar at 4508C prior to the deposition of the organometallic precursor. After deposition, the rhodium metallic particles were obtained by thermal decomposition of the entrapped precursor in ¯owing H2 from room temperature to 5008C with a heating rate of 108C/min [23]. The deposition of tin on rhodium metallic particles [Rh/NaY(IE) or Rh/NaY(CVD)] was accomplished by chemical vapour deposition of Sn(C2H5)4 or Sn(C6H5)4 in ¯owing argon at 558C and 2358C, respectively. The resulting material was decomposed in ¯owing hydrogen from room temperature to 5008C with a heating rate of 58C/min. The rhodium loading determined by AAS after chemical dissolution [24] was 10.05 wt% in all the samples. The Sn content, determined by ICPOES, was instead not constant for all samples and it will be speci®ed in the text as Sn/Rh molar ratio. 2.2. Temperature programmed studies The kinetics of thermal decomposition of tin precursors on Rh/NaY samples was studied by temperature programmed reductive decomposition (TPRD) in ¯owing H2 (8%)/He mixture from room temperature to 5008C with a heating rate of 38C/min. All samples coming from the decomposition studies were calcined at 5008C in ¯owing O2 and a temperature programmed reduction (TPR) analysis was then done in ¯owing H2 (8%)/Ar mixture from room temperature to 5008C with a heating rate of 88C/min. Full details for both techniques have been described previously [25,26]. 2.3. Hydrogen chemisorption The amounts of H2 adsorbed on the catalysts were measured in the pulse mode using a Micromeritics Pulse Chemisorption 2700 apparatus. Chemisorption measurements were carried out at 758C with H2 pulses
43
of 50 ml. The chemisorption temperature was determined after a temperature hydrogen desorption measurement from ÿ508C to 5008C. More detailed information about apparatus and procedures are reported elsewhere [25]. 2.4. Diffuse reflectance infrared spectroscopy (DRIFTS) In situ diffuse re¯ectance infrared spectra were recorded on a FTS-40 Digilab FT-IR spectrophotometer equipped with a Harrick Scienti®c DRA3CO diffuse re¯ectance accessory. A high pressure, high temperature environmental chamber for DRIFT studies (Harrick Scienti®c mod. HVC-DR3) allowed spectra of samples in powder form to be recorded in ¯ow conditions under controlled pressure and temperature. After TPRD investigation, the samples were transferred in air into the DRIFT cell and then reduced for 1 h in H2 ¯ow at 5008C (heating rate 108C/min) and cooled to room temperature in N2 ¯ow. Carbonylation was done in a pure CO ¯ow (6 ml/ min) at atmospheric pressure and room temperature. For every spectrum, 100 scans were collected at 4 cmÿ1 resolution. The interferograms were converted by means of the Kubelka±Munk transformation and plotted against wavenumbers. 2.5. Catalytic studies The hydrogenation of citral (mixture of cis- and trans-isomers, Aldrich C8,300-7, purity 95%) has been carried out in a 100 ml glass batch reactor thermostatted at 358C under H2 atmosphere. These very mild conditions were chosen to avoid diffusional limitations [27]. Typically, 20 ml of a 1:200 (v/v) citral:ethanol solution was loaded into the reactor; the amount of catalyst was such to have a substrate:rhodium molar ratio of about 200. The reaction was followed by analysing a suf®cient number of microsamples. Quantitative analysis was performed with a Helwett-Packard MSD 5971 GC± MS apparatus equipped with a DB-225 J&W column (0.23 mm i.d.30 m length). Catalytic conversions are calculated on citral consumption. Yields to cis- and trans-3,7-dimethyl-2,6octadien-1-ol (unsaturated alcohols ± UOLs) are
44
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
reported as (moles of UOLs)/(total moles of citral reacted)100. Yields to 3,7-dimethyl-6-octenal (saturated aldehyde ± SAL) and to acetal are reported similarly. To avoid the formation of the dietoxyacetal, some catalytic tests were carried out in n-heptane and nhexane. However, with these solvents, diffusion of reactants and products inside the zeolitic channels becomes the real rate determining step. 3. Results and discussion Cage-entrapped rhodium particles with or without intrazeolitic protons were obtained by ion-exchange (Rh/NaY(IE)) or by chemical vapour deposition (Rh/ NaY(CVD)), respectively. On these starting materials, organometallic tin precursors were then deposited by CVD. 3.1. Thermochemical characterisation Temperature programmed reductive decomposition (TPRD) analyses were carried out to study the decomposition of tin precursors as a function of the presence of protons and of the nature of tin complexes. In Fig. 1, the TPRD pro®les of Sn(C2H5)4 deposited on Rh/NaY(CVD) and on Rh/NaY(IE) are reported. On Rh/NaY(CVD) (Fig. 1(b)), the most abundant decomposition product is ethane, formed by b-elimination in H2 atmosphere: Rh0x -Sn C2 H5 4 =NaY2H2 ! Rh0x -Sn=NaY 4C2 H6 (1) The presence of two overlapped evolution peaks with maxima at 1608C and 2008C suggests a 2-step mechanism for the stoichiometric reaction (1). The intermediate formation of Sn(C2H5)x (x2) fragments supported onto the rhodium metal particle partially follows the ®ndings of Didillon et al. [16] for tetrabutyl tin on Rh/SiO2. The authors were in fact able to obtain stable Rh±Sn(alkyl)2 surface organometallic species by stopping the reduction process around 1308C: in our situation, the extensive overlap of the two decomposition steps does not allow the isolation of partially reduced Sn(C2H5)2 species, which rapidly decomposes just after formation.
Fig. 1. TPRD profiles of Sn(C2H5)4 deposited on: (A) Rh/ NaY(IE), Sn/Rh1.0; (B) Rh/NaY(CVD), Sn/Rh1.0; and (C) Rh/NaY(CVD), Sn/Rh2.0. Heavy line: m/z28, thin line: m/z15.
Methane evolution at 2808C indicates that a minor part of ethyl fragments are removed following a hydrogenolysis pathway assisted by the intrazeolitic rhodium metallic particles. A similar behaviour was already observed in the decomposition of Sn(Bu)4 on Rh/SiO2 [28], where ethane was evolved as a minor, high-temperature hydrogenolysis product.
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In the TPRD pro®le of Rh/NaY(IE) (Fig. 1(a)), where intrazeolitic protons were previously formed upon Rh(III) ion reduction, the methane evolution peak at about 2508C is far more pronounced than that obtained for the proton-free system. Moreover, on Rh/ NaY(IE), the evolution of ethane is now observed as a single and narrower peak at about 1808C. The intermediate formation of Sn(C2H5)x fragments, which are subsequently decomposed following a hydrogenolysis pathway, is thus strongly suggested. The increased methane evolution is now to be attributed to the hydrogenolytic removal of carbon residues by carbenium ion reactions of ethyl fragments in the presence of protons [29]. An increase in the amount of tin precursor (Sn/Rh molar ratio of about 2) causes the appearance in the TPRD pro®le (Fig. 1(c)) of a new and narrow peak of ethane at about 908C. Such evolution is consistent with the thermal decomposition of some Sn(C2H5)4 without any direct interaction with Rh particles. Subsequent experiments have shown that the narrow peak at 908C appears only when the Sn/Rh molar ratio exceeds 1.2±1.3. Since it is known that increasing the amount of tin precursor leads to a gradual coverage of the rhodium phase [30,31], our data strongly suggest that a complete saturation of the rhodium sites is reached at high Sn/Rh values. In other words, Sn(C2H5)4 is ®rst chemisorbed onto Rh particles and after that, a multilayer adsorption is taking place. This conclusion is also enforced by the observation that we were not able to deposit the tin precursor in the absence of the preformed intrazeolitic rhodium particles. The use of the Sn(C6H5)4 precursor leads to completely different results, owing to its high thermal stability requiring a sublimation temperature about 1808C higher than that of Sn(C2H5)4. The decompositions of this tin precursor deposited on Rh/NaY(IE) and on Rh/NaY(CVD) are shown in Fig. 2. In both cases, the only relevant peak concerns methane evolution around 3008C: no peaks at m/z77, diagnostic of benzene desorption, were observed. Noteworthy, the close similarity of the two pro®les of Fig. 2 clearly indicates that the reactivity is independent from the presence of protons, and hence that hydrogenolysis is the only decomposition process taking place. To our opinion, the high thermal stability of Sn(C6H5)4 could explain why a reductive elimina-
45
Fig. 2. TPRD profiles of Sn(C6H5)4 deposited on: (A) Rh/ NaY(IE), Sn/Rh1.0; and (B) Rh/NaY(CVD), Sn/Rh1.0. Heavy line: m/z77, thin line: m/z15.
tion pathway similar to reaction (1) is non-operative in this case: the hydrogenolysis pathway is in fact thermodynamically strongly favoured at high temperatures. From the TPRD studies, two observations can be drawn: 1. the decomposition of tin precursors takes place on the surface of the small rhodium particles only if the monolayer capacity is not overcome; and 2. the mechanism of decomposition is influenced by the presence of protons and by the tin precursors used. The TPR technique has been used in order to get information about the nature of the entrapped bimetallic particles and hence on the extent of the rhodium± tin interactions. The TPR pro®les of both Rh/NaY(CVD) and Rh/ NaY(IE) monometallic catalysts are well described in
46
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
the literature [32,33]. For the CVD sample, a simple reduction peak at 608C, assigned to the reduction of Rh2O3 to Rh0, is observed. In the case of IE sample, two peaks at 608C and 2708C are usually found. The low temperature peak is attributed to the reduction of Rh2O3 and/or RhO (being formed by reaction of Rh0 with protons), while the other one to the more thermodynamically stable Rh3 cations located in the super and/or in the sodalite cages of the NaY, that can only be produced in the presence of H: Rh2 O3 6H ! 2Rh3 3H2 O
(2)
Rh0 3H ! Rh3 3=2 H2
(3)
The reduction pro®le of a Sn/NaY prepared by ion exchange from SnCl2 (owing to the impossibility of preparing Sn/NaY samples by CVD) is shown in Fig. 3(a): a broad peak that starts at about 3508C but is still not complete at 5008C is observed. More-
Fig. 3. TPR profiles after calcination at 5008C: (A) Sn/NaY(IE); (B) Sn±Rh/NaY(IE) obtained from Sn(C6H5)4 (Sn/Rh0.38); (C) Sn±Rh/NaY(IE) obtained from Sn(C2H5)4 (Sn/Rh0.57); and (D) Sn±Rh/NaY(IE) obtained from Sn(C6H5)4 (Sn/Rh2.0).
over, the total amount of hydrogen consumed up to 5008C can only account for a partial reduction of tin. A similar behaviour was observed for Mn/NaY samples prepared by ion exchange, where no reduction peaks were observed at all: in that situation, the segregation of the manganese cations into the small sodalite cages was strongly suggested [33,23]. The high ®eld stabilisation exerted by sodalite cages onto bivalent cations could then be invoked also in this case to explain the strong tendency of tin to remain in the oxidised state. When tin is deposited by CVD onto pre-reduced Rh/NaY(IE), the TPR pro®les signi®cantly change. As it can be seen in Fig. 3(b) and (c), the reduction maxima are located at exactly the same position independently from the nature of the tin precursor: two distinct low temperature reductions are observed at 608C and 808C, with two other broader peaks at 2308C and 2708C. In both the cases, the total amount of hydrogen consumed accounts for the complete reduction of SnIV to Sn0 and of RhIII to Rh0. It is noteworthy that the amount of H2 consumed below 1508C is much higher than that required for the total rhodium reduction, meaning that a partial reduction of tin is also involved in this region. Moreover, the peak at 808C shows a marked increase passing from a Sn/Rh value of 0.38 to 0.57 and ®nally to 2.0 (Fig. 3(b)±(d)). This peak could then be unambiguously assigned to the reduction of a SnOx phase. As in the monometallic rhodium catalyst, the peak at 608C is instead attributed to the reduction of Rh2O3 and/or RhO. Similarly (again compare Fig. 3(b)±(d)), the two high temperature peaks at 2308C and 2708C are, respectively, assigned to the reduction of Snn and Rh3 ionic species, being formed by the reaction of the oxidised phases with intrazeolitic protons. The enhanced reducibility of Sn in the presence of Rh particles, as previously observed for many bimetallic catalysts in zeolite [34], is a direct indication of the intimate contact of the two metals in the bimetallic catalyst. Another interesting feature of the high tin loaded TPR pro®le (Fig. 3(d)) is the relevant hydrogen consumption above 3508C. This last feature closely resembles the behaviour of Sn/NaY, which indicates that at least a part of tin is no more in contact with the
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47
Table 1 Hydrogen chemisorption data Catalyst
Precursor
Sn/Rh
H/Rh
Sn/Y(IE) Rh/NaY(CVD) Rh/NaY(IE) Sn±Rh/Y(IE) Sn±Rh/Y(IE) Sn±Rh/Y(CVD) Sn±Rh/Y(CVD)
SnCl2 Rh(acac)(CO)2 [Rh(NH3)5Cl]2 Sn(C6H5)4 Sn(C2H5)4 Sn(C6H5)4 Sn(C2H5)4
± ± ± 0.38 0.57 0.49 0.51
0 0.92 1.04 0.67 0.56 0.40 0.70
3.2. Chemisorption measurements
Fig. 4. TPR profiles after calcination at 5008C: (A) Sn±Rh/ NaY(CVD) obtained from Sn(C6H5)4 (Sn/Rh0.49); and (B) Sn± Rh/NaY(CVD) obtained from Sn(C2H5)4 (Sn/Rh0.51).
rhodium phase. These results are in agreement with the TPRD data observed when the Sn/Rh molar ratio of about 1.2±1.3 is overcome (compare Fig. 1(b) and (c)). The TPR pro®les of the proton-free Sn±Rh/ NaY(CVD) samples are reported in Fig. 4. As in the case of the related IE systems, the amount of hydrogen consumed accounts for the complete reduction of SnIV to Sn0 and of RhIII to Rh0, both for the Sn(C2H5)4 and for the Sn(C6H5)4 based samples. Moreover, recalling that in the absence of intrazeolitic protons, no ionic Rh3 or Snn species can be present, all TPR peaks have to be attributed to neutral oxidised species only. However, the TPR pro®les are now much more complex than those of the Sn±Rh/NaY(IE) samples, and do not allow a clear identi®cation of the reduction processes being involved. Experiments carried out changing the Sn/Rh molar ratio have shown a great variability both of peaks height and of peaks position, indicating that in the absence of protons, the nature of the rhodium±tin phase is also composition-dependent.
The hydrogen chemisorption measurements carried out in the pulse mode on mono- and bimetallic catalysts are reported in Table 1. Both IE and CVD monometallic Rh/NaY catalysts show H/Rh values near unity that, according to a 1:1 H:M stoichiometry [35], indicate a good dispersion level. Moreover, tin does not chemisorb H2, as previously reported [36]. The bimetallic catalysts show chemisorption values lower than their respective Rh/NaY systems. Since it is reasonable to suppose that inside the zeolite cages, the dimensions of the rhodium particles do not increase upon deposition of the tin phase, the lower H/Rh values reported here have to be attributed to a partial tin coverage of the exposed rhodium surface. The CVD-based Sn±Rh/NaY samples show different H/Rh chemisorption values as a function of the tin precursor. The chemisorption data indicate a more pronounced tin coverage for the Sn(C6H5)4 derived catalysts than for the Sn(C2H5)4 one. Our data would suggest that the utilisation of Sn(C6H5)4 leads to the formation of bimetallic particles with a higher tin coverage than that obtained from Sn(C2H5)4. Additional support to this suggestion is provided by our IR spectra. The two bimetallic catalysts derived from Rh/ NaY(IE) samples show instead similar dispersion, with the higher H/Rh value observed for the sample with the lowest Sn/Rh ratio. According to the TPR measurements, these chemisorption data con®rm that, in the presence of protons, the nature of the entrapped rhodium±tin particles does not depend upon the tin precursor.
48
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
Fig. 5. DRIFT spectra of Sn±Rh/NaY(IE) obtained from Sn(C6H5)4: (A) after 180 min in flowing CO at room temperature; and (B) from (A) after 30 min of purging in Ar flow at room temperature.
3.3. DRIFT spectra of adsorbed CO Diffuse re¯ectance infrared FT (DRIFT) spectra of the Sn(C6H5)4-derived Sn±Rh/NaY(IE) sample (Fig. 5) after carbonylation are substantially similar to those obtained with monometallic Rh/NaY(IE) [37]. In that case, the bands above 2000 cmÿ1 were attributed to the presence of two Rh(I) gem-dicarbonyl species [32,38], while the partial formation of Rh4(CO)12, with its doubly bridged CO ligands, is responsible for the 1830 cmÿ1 absorption [39]. This latter species is stable only in CO atmosphere and tends to reorganise to the more stable Rh6(CO)16 when
CO is purged away: during the interconversion a rapid decrease of the 1834 cmÿ1 is observed, which parallels the raise of the 1763 cmÿ1 band, due to triply bridged CO ligands [23]. As it can be seen in Fig. 5, similar changes are observed for Sn±Rh/NaY(IE), meaning that this catalyst closely resembles the behaviour of the monometallic Rh/NaY(IE) system. Since from the TPR analysis, we have unambiguously concluded that the easy reducibility of tin has to be ascribed to the intimate contact with the Rh particles, it seems that such contact cannot be observed by IR analysis. However, we have to consider that in the presence of CO and protons, an oxidative disruption process of Rh particles occurs [38], causing the formation of Rh CO 2 and the subsequent removal of Rh±Sn interactions. From this point of view CO does not properly act as a probe molecule, since in the presence of protons it is able to modify the nature of the entrapped particles. Clear evidences of bimetallic Rh±Sn interactions are observed only with CVD catalysts, as the oxidative disruption process cannot be active not having protons. Signi®cant differences are in fact observed when the DRIFT spectra of Fig. 6(c) and (d) (relative to CVD-based catalysts obtained using Sn(C6H5)4 and Sn(C2H5)4, respectively) are compared with those of the monometallic Rh/Y(CVD) catalyst (Fig. 6(a) and (b)). Bands due to terminal CO ligands between 2100 and 2000 cmÿ1 differ substantially from the ones of the Rh/NaY(CVD) sample. On the contrary, the band at 1830 cmÿ1 is observed even for both bimetallic samples. However, differently from the monometallic sample [23], this band does not disappear when CO is removed by ¯owing inert gas. On this basis, we would reject the attribution of the 1830 cmÿ1 band to the formation of Rh4(CO)12, since its transformation into the hexanuclear cluster is a facile process, being always observed both in homogeneous [40] and in heterogeneous phase [41]. The double bridging coordination of CO may result from a preferential attachment of tin onto the three-fold hollow sites of the pre-formed rhodium particles, preventing CO to coordinate in the triply bridging form. The preferential location of tin on the more coordinatively unsaturated rhodium sites was in fact previously reported by Coq et al. [31]. It is noteworthy that the total amount of adsorbed CO decreases passing from Sn(C2H5)4 to Sn(C6H5)4-
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
49
dimethyl-2,6-octadienal). The catalytic data are summarised in Table 2. With the monometallic Rh/NaY(IE) catalyst, we observe both the formation of the diethyl acetal, and the hydrogenation of the conjugated C=C double bond to give the saturated aldehyde citronellal (SAL). Instead, the proton-free Rh/NaY(CVD) catalyst does not generate any signi®cant amount of acetal, with the saturated aldehyde being the only reaction product. Hence, the presence of intrazeolitic protons is highly detrimental, effectively catalysing the acetal formation. On the bimetallic Sn±Rh/NaY(IE) catalysts, the yield of the acetal is much higher than that of the monometallic Rh/NaY(IE) system, with citronellal being only a minor product. The additional presence of Sn2 Lewis acid sites, formed through one of these pathways, Sn0 2H ! Sn2 H2 Sn0 H2 O ! SnO H2 # SnO 2H ! Sn2 H2 O
Fig. 6. DRIFT spectra of CVD-based mono- and bimetallic catalysts: (A) Rh/NaY(CVD) after 180 min in flowing CO at room temperature; (B) from (A) after 30 min of purging in Ar flow at room temperature; (C) Sn±Rh/NaY(CVD) obtained from Sn(C6H5)4 after complete carbonylation and subsequent purging in Ar flow; and (D) Sn±Rh/NaY(CVD) obtained from Sn(C2H5)4 after complete carbonylation and subsequent purging in Ar flow.
derived catalysts (compare Fig. 6(a) and (b)). In agreement with the conclusions gained on the basis of thermochemical and chemisorption data, we may con®rm that the latter sample shows a more pronounced tin coverage. 3.4. Catalytic activity The possibility of modelling rhodium±tin interactions has been monitored by testing our bimetallic catalysts in the selective hydrogenation of citral (3,7-
(4) (5)
easily accounts for the catalytic condensation to acetal and was also already reported by Galvagno et al. [42] on Ru±Sn catalysts for the selective hydrogenation of cinnamaldehyde. The utilisation of a reacting solvent like ethanol plays thus a key role allowing to detect the presence of Brùnsted and Lewis acid sites. In fact, as discussed above for the Rh/NaY(CVD) system, the formation of acetal is completely inhibited also with the proton- and Lewis acid-free bimetallic Sn±Rh/ NaY(CVD) catalysts. Moreover, a competition towards the attack of conjugated C=C and C=O is instead observed with the bimetallic systems. In this case, the major reaction products are the saturated aldehyde and the two unsaturated alcohols, cis- and trans-3,7-dimethyl-2,6-octadien-1-ol (UOLs), deriving from cis- and trans-citral. As has been reported for tin promoted catalysts [4], the hydrogenation of C=O requires an activation time: during this period, the reaction of the conjugated C=C is quite fast and decreases in favour of C=O hydrogenation. The active surface must hence experience a partial modi®cation to become active towards the hydrogenation of C=O. Since in the starting catalyst both metals are in the zero-valent state, we have to conclude that, as
50
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
Table 2 Catalytic data for citral hydrogenation Catalyst Rh/NaY(IE)a Rh/NaY(CVD)a Sn±Rh/NaY(IE)a Sn±Rh/NaY(IE)a Sn±Rh/NaY(CVD)b Sn±Rh/NaY(CVD)b a b
Tin precursor
Conversion (%)
Sn(C6H5)4 Sn(C2H5)4 Sn(C6H5)4 Sn(C2H5)4
60 63 63 62 45 47
Product distribution (%) Acetal
SAL
UOLs
56 ± 88 86 ± ±
43 98 12 14 92 61
± ± ± ± 8 39
After 1400 min. After 2500 min.
reported for Rh±Sn/SiO2 catalysts [15], a Rh0±Sn0 phase is reasonably not active towards the C=O hydrogenation. A partial oxidation of the tin phase is strongly suggested to occur during the induction period, as reported by Marinelli and Ponec [5]. However, we have to exclude the presence of ionic Sn2 because (i) no acetal is formed with these catalysts, and (ii) the charge balance of the zeolite does not allow the formation of any isolated ion. The presence of a neutral tin oxidised phase (SnOx) is then the most favoured as the actual promoter for C=O bond activation. 4. Conclusions There are two strategies to design the rhodium±tin interactions, namely (i) the introduction of surface protonic acidity, and (ii) a change in the reactivity of the volatile tin precursor. The presence of intrazeolite protonic acidity is shown to promote Sn2 formation either by reoxidation of the zerovalent Sn phase or by acid±base reaction with SnO (or SnO2) formed by reaction with water (reactions 4 and 5). The effect of the presence of protons is even more dramatic in CO-containing atmosphere, since the oxidative disruption of Rh particles to Rh CO 2 , totally destroys Rh±Sn interactions. It is thus possible to progressively shift from a true Rh0±Sn0 phase to Rh±Sn (2) as a simple function of the proton content of the zeolite structure. In this respect, the use of chemical vapour deposition might be considered far superior to ion-exchange, since the H/metal ratio is
no more limited by the stoichiometry of ion reduction, but it can be freely varied by simply substituting the alkaline (Na or K) compensating cations with protons in the starting zeolite [14]. In proton-free environments, the intrinsic thermal stability of the tin alkyl precursor may also in¯uence the topology of the bimetallic Rh±Sn particles. At low temperatures (below 2008C), decomposition of the tin alkyl precursor tends to follow a two-step SnR4 ! SnRx ! Sn0 mechanism onto speci®c sites, probably three-fold hollow sites, of the entrapped Rh clusters, leading to the formation of bimetallic Rh0±Sn0 particles. The high temperature decomposition of a stable tin precursor such as Sn(C6H5)4 occurs instead preferentially by the hydrogenolytic rupture of Sn±R bonds, and results in surface-tin-enriched, cherry-type particles. As we have observed with chemisorption and DRIFT analysis, the tin surface coverage is in fact much more pronounced when Sn(C6H5)4 instead of Sn(C2H5)4 is used. Differences in the decomposition pathways are thought to be responsible for these coverage dissimilarities. With Sn(C2H5)4, the multi-step decomposition mechanism necessarily implies that the tin precursor is partially decomposed on the just formed Sn0 species, even if the monolayer capacity of Rh0 particles is still not exhausted. This situation, represented by I in the scheme reported below, is similar to the model and its supporting arguments proposed by Didillon et al. [28] for tin promoted Rh/SiO2 systems. With Sn(C6H5)4, the evidence of a concerted decomposition process implies that the major part of tin precursor molecules reacts on super®cial rhodium atoms, leading to a fairly monolayered bimetallic particle, represented by model II:
S. Recchia et al. / Applied Catalysis A: General 182 (1999) 41±51
These models may account for the different rhodium coverage observed in the two cases. Moreover, since activity towards C=O hydrogenation can be related to the concentration of accessible rhodium± tin ensembles, the type I model may also account for the higher selectivity to UOLs observed for the promoted catalyst derived from Sn(C2H5)4. The catalytic tests afforded important information on the nature of the tin promotion effect. This is due to the presence of an oxidised tin non-ionic phase, formed under catalytic conditions, and operating through a Lewis acid±base interaction between the carbonyl group and the exposed tin atom at the edge of the oxide moieties. Acknowledgements We gratefully acknowledge ®nancial support from National Research Council, and Ministry of University and Scienti®c and Technological Research. References [1] J.L. Margitfalvi, M. HegeduÈs, E. TaÂlas, J. Mol. Catal. 51 (1989) 279. [2] K. Tomishige, K. Asakura, Y. Iwasawa, J. Catal. 149 (1994) 70. [3] R.L. Augustine, L. Meng, in: R.E. Malz Jr. (Ed.), Catalysis of Organic Reactions, Marcel Dekker, New York, 1996, p. 15. [4] V. Ponec, Appl. Catal. A 149 (1997) 27. [5] T.B.L.W. Marinelli, V. Ponec, J. Catal. 156 (1995) 51. [6] T.B.L.W. Marinelli, J.H. Vleeming, V. Ponec, in: Proceedings of the 10th International Congress on Catalysis, Vol. B, Budapest, 1992, p. 1211. [7] Yu.I. Yermakov, B.N. Kuznetsov, J. Mol. Catal. 9 (1980) 13. [8] J.L. Margitfalvi, E. TaÂlas, S. GoÈboÈloÈs, Catal. Today 6 (1989) 73. [9] J.P. Candy, A. El Mansour, O.A. Ferretti, G. Mabilon, J.P. Bournonville, J.M. Basset, G. Martino, J. Catal. 112 (1988) 201. [10] J.P. Candy, O.A. Ferretti, G. Mabilon, J.P. Bournonville, A. El Mansour, J.M. Basset, G. Martino, J. Catal. 112 (1988) 210. [11] M. Agnelli, P. Louessard, A. El Mansour, J.P. Candy, J.P. Bournonville, J.M. Basset, Catal. Today 6 (1989) 63.
51
[12] O.A. Ferretti, C. Lucas, J.P. Candy, J.M. Basset, B. Didillon, F. Le Peltier, J. Mol. Catal. 103 (1995) 125. [13] M. Bellatreccia, R. Zanoni, C. Dossi, R. Psaro, S. Recchia, G. Vlaic, J. Chem. Soc., Faraday Trans. 91(13) (1995) 2045. [14] C. Dossi, R. Psaro, R. Ugo, Z.C. Zhang, W.M.H. Sachtler, J. Catal. 149 (1994) 92. [15] B. Didillon, A. El Mansour, J.P. Candy, J.P. Bournoville, J.M. Basset, Stud. Surf. Sci. Catal. 59 (1991) 137. [16] B. Didillon, J.P. Candy, A. El Mansour, C. Houtmann, J.M. Basset, J. Mol. Catal. 74 (1992) 43. [17] B. Didillon, J.P. Candy, F. Le Peltier, O.A. Ferretti, J.M. Basset, Stud. Surf. Sci. Catal. 78 (1993) 147. [18] D.G. Blackmond, R. Oukaci, B. Blanc, P. Gallezot, J. Catal. 131 (1991) 401. [19] P. Gallezot, A. Giroir-Fendler, D. Richard, in: W.E. Pascoe (Ed.), Catalysis of Organic Reactions, Marcel Dekker, New York, 1992, p. 1. [20] D.G. Blackmond, A. Waghray, R. Oukaci, B. Blanc, P. Gallezot, Stud. Surf. Sci. Catal. 59 (1991) 145. [21] W.M.H. Sachtler, Z.C. Zhang, Adv. Catal. 39 (1993) 129. [22] C. Dossi, R. Psaro, A. Bartsch, E. Brivio, A. Galasco, P. Losi, Catal. Today 17 (1993) 527. [23] T. Beutel, H. KnoÈzinger, H. TrevinÄo, Z.C. Zhang, W.M.H. Sachtler, C. Dossi, R. Psaro, R. Ugo, J. Chem. Soc., Faraday Trans. 90 (1994) 1335. [24] C. Dossi, P. Losi, S. Calmotti, Ann. Chim. 83 (1993) 233. [25] C. Dossi, A. Fusi, O. Russo, Analysis Europa, April 1995, p. 17. [26] C. Dossi, A. Fusi, R. Psaro, Thermochim. Acta 236 (1994) 165. [27] A. Waghray, R. Oukaci, D.G. Blackmond, Stud. Surf. Sci. Catal. 75 (1993) 2479. [28] B. Didillon, C. Houtmann, T. Shay, J.P. Candy, J.M. Basset, J. Am. Chem. Soc. 115 (1993) 9380. [29] P.J.M. Rek, A.J. den Hartog, V. Ponec, Appl. Catal. 46 (1989) 213. [30] J.P. Candy, B. Didillon, E.L. Smith, T.B. Shay, J.M. Basset, J. Mol. Catal. 86 (1994) 179. [31] B. Coq, A. Goursot, T. Tazi, F. FigueÂras, D.R. Salahub, J. Am. Chem. Soc. 113 (1991) 1485. [32] D.C. Tomczak, V.L. Zholobenko, H. TrevinÄo, G.-D. Lei, W.M.H. Sachtler, Stud. Surf. Sci. Catal. 84 (1994) 893. [33] C. Dossi, A. Fusi, S. Recchia, R. Psaro, Mater. Eng. 5 (1994) 249. [34] J.S. Feeley, W.M.H. Sachtler, J. Catal. 131 (1991) 573. [35] H. Hamada, R. Funaky, Y. Kuwahara, Y. Kintaichi, K. Wakabayashi, T. Ito, Appl. Catal. 30 (1987) 177. [36] H. Veerbeek, W.M.H. Sachtler, J. Catal. 42 (1976) 257. [37] M. Primet, J. Chem. Soc., Faraday Trans. I 74 (1978) 2570. [38] G. Bergeret, P. Gallezot, P. GeÂlin, Y. Ben Taarit, F. Lefebvre, C. Naccache, R.D. Shannon, J. Catal. 104 (1987) 279. [39] L.-F. Rao, A. Fukuoka, N. Kosugi, H. Kuroda, M. Ichikawa, J. Phys. Chem. 94 (1990) 5317. [40] P. Chini, S. Martinengo, Inorg. Chim. Acta 3 (1969) 315. [41] A. Theolier, A.K. Smith, M. Leconte, J.M. Basset, G.M. Zanderighi, R. Psaro, R. Ugo, J. Organometal. Chem. 191 (1980) 415. [42] S. Galvagno, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 8 (1991) 9.