- Email: [email protected]
Al2O3 heterogeneous catalysts using a diisocyanoligand as an integral design component
PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All rights reserved.
1085
Preparation of Rh-Co/AI203 heterogeneous catalysts using a diisocyanoligand as an integral design component M.S.W. Vong and P.A. Sermon Solids and Surfaces Research Group, Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH, UK Here consideration has been given to the potential for diisocyanide ligands to complex a transition metal (TM) cation to produce a monolayer of a highly-porous polymer in which more than 95% of metal atoms are constrained in well defined and characterisable active states with stable and known metal-metal coupling, symmetry and oxidation state. Catalysts prepared in this way have shown activities and selectivity in alkene hydrogenation, whereby the molecular network provides the shape-selectivity and the guest metal ion provides, the chemical reactivity and selectivity. This will probably lead to a new range of highly active and selective TM mono- and bi-metallic catalysts and adsorbents which could be of immense technical and scientific importance. 1. I N T R O D U C T I O N In recent years much attention has focused on the production of novel catalysts, adsorbents and sensors with high reactivity and selectivity9 Rh is a particularly interesting metal to consider in this context in that it is selective in alkene hydrogenation [1, 2], asymmetric hydrogenation [3] and CO/CO 2 hydrogenation [4]. In essence only a small fraction of Rh atoms in traditional supported catalysts are highly active [5] and their activity and selectivity are also affected by the fractal dimension of catalyst surfaces through the rates of reactants and products diffusion to and from the active sites [6]. Also immobilized and heterogenized transition metal complexes act as catalysts for homogeneous reactions and have become an area of intense catalytic interest. Others have used supported complexes, clusters [7] and polymers which are catalytically active. Molecular precursors may be one answer of particular relevance to highly dispersed Rh. The aim of this work was to use molecular receptors to produce, two- and three-dimensional networks into which a Rh reaction centre could be constrained so as to control their chemical nature and stereo environment to produce catalysts of high activity-selectivity in hydrogenation. In such an aproach to reactivity-control, bridging aryl-diisocyanide ligands (e.g. CNX-NC, where X is a benzene, diphenyl, etc group), are interesting in that (i) CN- groups chelate very effectively with metals via MdCxv)-to CN(2~) and CNfss* )to Md(x2_v2) overlap [8], producing net electron donation to Rh [9]. - (iO there is extended delocalisation in the x-y plane of the 'polymer' which is greatest when conjugation [10] is present and this produces substantial stability for the supported networks constraining the Rh +. (iii) metal-metal bonding is also allowable in the z-direction if multilayers are produced on a support (between the x-y laminae) since the sheets then stack up with metal centres aligned with weak dz2-dz2 a d Pz-Pz overlap. Thus in terms of metal-metal bonding and one-dimensional conductivity, ~ere are "similarities between the diisocyanocomplexes [11] and other one-dimensional conductors K2Pt(CN)4.Brx.3H20 [ 12].
1086 Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-1-enes over Wilkinson's catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh +] in benzene is quite selective (i.e. was 45 times faster than the cis-alk-2-ene [ 15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4'-diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [ 14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. Ligands such as the 1,4-diisocyanobenzene or 1,4-phenyl-diisocyanide have been described as rigid collinear bridging ligands, which separate Rh ions by 1.2nm [ 17]. This has been used exclusively here and is denoted L (e.g. the unsupported polymer is given by [RhL2] n if we ignore any halogen and water components). The activity in hydrogenation of butenes of oxide-supported Rh/Co 1,4-diisocyanobenzene complexes is compared with that for simple impregnated catalysts. The unsupported Rh-diisocyanobiphenyl polymers are quite stable to 469K-617K in H 2 [9] but turnover frequencies in cyclohexene hydrogenation 293K were 3000 times lower than for silica-supported Rh and also decreased rapidly with use above 363K; hence the approach here to use the supported polymers. 2. EXPERIMENTAL 1,4-diisocyanobenzene (L) (Aldrich, 99% purity); [Rh(Co)2C1]2 (Aldrich; >97% purity), CoC12 (Fisons, >97% purity), A1203 (Degussa; 99% purity) and SiO 2 (Davison 923) were used.
2.1. Preparation 1,4-diisocyanobenzene undergoes a facile reaction with tetracarbonyl-bis-lxchloridedirhodium to produce a bulk tetragonal Rh-l,4-diisocyanobenzene chloride polymer with a [RhL2+.C1-.xH20] network. 2(CN-~-NC) + 1/2[Rh(CO)2C112
~. [Rh(CN-~-NC)2]+.CI'.x-H20 + 2CO
(1)
Here supported complexes were prepared by first impregnating the oxide with 5% w/w or 10% w/w ligand from a dichloromethane solution and then drying this under vacuum at room temperature. This was then allowed to exchange with a solution of [Rh(CO)2C1] 2 or a mixed solution of [Rh(CO)2C1]2 and CoC12 in ethanol (in which L was insoluble). From this the supported complex formed very quickly. After washing with fresh solvent (to remove unreacted chloride and ligand), the sample was then evacuated to dryness at room temperature.
2.2. Catalyst characterisation AA spectroscopy was used to study the rate and extent of metal incorporation under the above conditions. Powder X-ray diffractometry (Philip PWl710), FTIR (Perkin Elmer 1710), diffuse reflectance spectroscopy (Perkin Elmer Lamda 9 uv-vis near ir photospectrometer), XPS (Kratos ES300 with A1 radiation at 1468.6 ev), ESR (Varian E3 calibrated with DPPH) and TEM (Joel Instruments CX100) were used to confirm the formation of this ordered polymeric Rh + network on the oxide surface. TPR and in-situ FTIR were also used to study the thermal stability of the supported and unsupported complexes in H 2, CO and air. The total surface area of the oxide-supported catalysts was measured by BET N2 adsorption (Carlo Erba Sorptomatic 1800) at 77K.
1087
2.3. Catalytic activity measurements The activity of the catalysts in alkene hydrogenation was measured by passing a reactant stream (1.24kPa butene, cyclohexene or benzene, 76kPa H 2 and N 2 (balance to 101kPa)) through a catalyst sample (0.02-0.05 g) at 30 cm 3 rain "1 and 295-6K. Reaction products were analysed as a function of time by FID gas chromatography. Before reaction the sample were pretreated in hydrogen (393K) and then flushed with N 2 while cooling to the reaction temperature. 3. RESULTS
3.1. Characterisation AA showed that when the metal to ligand ratio (M/L) added was 0.5, 99% of the Rh and 80% of the Co was taken up rapidly by the A1203-supported diisocyanobenzene (compared to 20% and 10%, respectively without L) and there was also lower extents of leaching of A13+ from the support when L was present: Table 1 Metal concentrations in solution (ppm) measured by AA analysis .. L/Alumina
0 1 2 3 10
Alumin0t
[Rh] [Co] [AI]
[ma] [Co] [A1]
16.0 1.5 2.1 1.6 1.8
16.0 12.2 11.0 12.7 12.1
9.0 0.2 0.2 0.1 0.1
0.4 7.1 0.8 0.0 0.2
9.0 4.0 9.2 124.1 7.5 96.7 8.3 78.5 8.5 182.5
X-ray diffraction patterns obtained for the unsupported and oxide supported Rh complex corresponded to a crystalline network of tetragonal structure [14, 18] and FTIR spectroscopy indicated (see Figure 1) that the oxide supported complex is similar to the unsupported polymers [ 19] with absorption corresponding to the aromatic ring around 260 nm and the strong CN stretching frequencies at about 2120-2160 cm -1 being retained, although there is probably evidence of decrease conjugation in the oxide-supported state. Spectra of L/silica and L/alumina showed only a weaker CN absorption band indicating that the diisocyanobenzene ligand also complexed to a smaller extent with the A13+ or the Si 4+ ions on the surface of the catalyst supports. Interestly this absorption band was not observed for the 1,4-diisocyanobenzene ligand alone. The CN band of the diisocyano-network is relatively constant in oxidised or reduced atmosphere at 378K and when used in but-l-ene hydrogenation at 338K. UV-Vis diffuse reflectance spectra of the Rh/L polymer showed one intense absorption maximum at--400nm, close to that observed with Rh(CNPh)4BPh 4 monomer [20] and a board absorption band centred at 700nm-750nm, which is characteristic of weak intrachain Rh ....Rh interaction in polymers [18, 21] such as {Rh(diisocyanide)2+C1-]n. These electronic absorptions were asigned to the transition between energy levels in the A lz(dz2) bonding and A2u(p2p*) antibonding orbitals. The intensity of the above peaks decrease~l for
1088
d[H]/dt
% Transmission
b
(c) 3200
2400 1800 Wavenumber (cm -1)
Figure 1. FTIR spectroscopic properties of 3%Rh/0.6%Co/L/A120 3 (a), 10% L/A1203 (b) and A1203 (c). (*) denotes the CN stretching band of the diisocyano-network.
273
I
I
I
I
473
673
873
1073
T (K) Figure 2. TPR profile of 2%Rh/1.5%Co/ 10% L/A1203 (a), 2%Rh/L/A1203 (b) and unsupported Rh/L (c).
Figure 3. TEM micrograph of 4%Rh/L/A120 3 (x 893,000).
1273
1089 polymers in the supported state, which suggested a lower degree of metal-metal interaction (i.e. a decrease in the conjugation of the network and a resulting lower stability). The presence of Co in the complex increased the intensity of the absorption band and shifted it to longer wavbelength. Photoelectron spectroscopy revealed binding energies for Rh 3d5/2__(308.4eV) in the alumina-supported diisocyano network, which is well above the value (307.7eV) seen for Rh ~ The binding energies measured for Co 2pl/2 suggested that the Co was also in positive oxidation state. The constrained metal centres were stabilized by the extended conjugated ligand networks via electron delocalisation in the x-y plane. TG/DTA confirmed that Rh/Co/L/A120 3 was stable to at least 543K in H 2, with an exothermic reaction at only 581K. TPR profiles for the rhodium 1,4-diisocyanobenzene polymer are shown in Figure 2. The unsupported rhodium complex reduced at higher temperature (as much as 250K) than the Rh-Co/A120 3 impregnated catalyst. This was direct evidence of the stabilising effect of the polymeric network. The first reduction peak at low temperature was related to the liberation of chlorine ions as HC1 and the reduction of the rhodium centres (Rh:H = 1:4, after allowing for HC1 liberation). Hydrogen uptake at high temperature is believed to correspond to the degradation of the polymer matrix (L:H= 1:6). When the complex was supported on oxides, it became less stable and reduction occurred at lower temperature. Co incorporation increased the stability of the alumina-supported Rh network and raised the reduction temperature by 30K. The Rh-Co/L/A120 3 catalyst also showed an improved catalytic activity in alkene hydrogenation. Total surface areas of the catalysts were determined by BET N 2 adsorption at 77K and X-ray diffraction line broading (XRDLB). Table 2 Total surface areas (S) and average crystallite sizes (d) of the Rh complex catalysts* S (m2 g-l) AhO 3 (C) 100 L/A120 3 125 1%Rh/L/A120 3 144 4%Rh/L/A120 3 170 RhL 2 90 3 % Rh/0.575 % Co/L/A1203 109 2%Rh/1.15 %Co/L/A1203 107 (CoL was amorphous and showed no structure which was
d (nm)
47.9 43.3 41.2 14.9 detectable by XRD).
* deduced from BET and XRDLB at 7.8~ The Rh-1,4-diisocyanobenzene polymer was highly porous and revealed a Type II N 2 adsorption isotherm. It possessed an extensive rnesoporous structure and a zeolite-like microproous structure. The the total surface area of the non-porous A120 3 support was increased by L and metal-L complexation. Thus this increase in porosity suggested that these materials could be used as a selective adsorbents. Transmission electron micrograph (Figure 3) of the alumina supported Rh-diisocyano complex shows semicrystalline layer structure and unlike the impregnated catalyst, no Rh particles were observed.
3.2. Catalytic activity In Figure 4 the activity in hydrogenation of butenes of this alumina-supported 1,4diisocyanobenzene-constrained Rh/Co catalyst is shown and is compared to the characteristic
1090
nmol/g cat./min.
O
60
O
O
(a)
O 40 20 A
ID (b) 800 6OO
-
~
O-," I v
9
o
~
o
""
200 1
0
I
I
20
! 40
I
I
I
60
t (min)
Figure 4. Activities of 3%Rh/0.6%Co/I.JA120 3 (a) and 3%Rh/0.6%Co/A1203(b ) in hydrogenation of but-1-ene (O), cis-but-2-ene (e) and trans-but-2-ene (0) at 295K.
b
P
Y Figure 5. Manner of adsorption of cis-but-2-ene ( ~ ) on Rh ( ~ ) (constrained by diisocyanobenzcne in the x-y plane) along the z-axis where steric control depends upon alignment of aromatic rings and this will depend upon interactionswith the oxide support. The structuregiven is also thatseen for unsupported networks.
1091 of those of the corresponding impregnated materials. F~st the rates of butene hydrogenation over both catalysts are reasonabUy stable with reaction time. Second, unlike the impregnated catalyst, the supported polymer did shows stereoselectivity with decreasing rates in the sequence: but-1-ene > cis-but-2-ene > trans-but-2-ene even though the apparent hydrogenation activity of the supported polymer was 104-105 folds lower than the conventional Rh-Co/A12Oy For but-l-ene hydrogenation there was no significant isomerisation to but-2-ene isomers for the alumina-supported polymer, but only for the impregnated catalyst (as seen previously [22]). Others have found that the cis:trans ratio in isomerisation products accompanying alkene hydrogenation was very variable [23]. 4. DISCUSSION Often homogeneous catalysts are more selective (although more subject to diffusionlimitation) than heterogeneous ones [15]. Attempts to induce higher selectivity on metal active centres in heterogeneous catalysts are important in that the products would not be subject to the diffusion limitation to the extent that homogeneous catalysts are. In addition, immobilization of homogeneous catalysts using ligands would eliminate the difficulties in separating catalysts from reactants and/or products and allow catalysts to be regenerated and in some cases, reduce rate of observed catalyst deactivation. It appears that one mode of steric and/or electronic control is via a stabilising network to constrain the metal ions [22] using pre-adsorbed bridging aryl-diisocyano ligands which (i) chelate very effectively, (ii) have a stability enhanced by extended delocalisation and conjugation in the x-y plane, and (iii) allow metal-metal bonding in bulk networks in the zdirection (between the x-y laminae). I n the unsupported state such polymers do not appear to show this selectivity. At 298K and 2 atmos H 2 the Pd 0 complex of 4,4'diisocyanobiphenyl catalysed the hydrogenation of hex-1-ene to n-hexane [9, 14] but also produced trans- and cis-hex-2-ene in the ratio 4 (as did a 10%Pd/C catalyst) by isomerisation. However, the precise product ratio varied with time in a manner reminiscent of an alkene titration [24] with isomerisation at intermediate times. It may be that with such complexes the H concentration at Pd centres changes with the approach used on but-l-ene titration was to assume that hydrogenation and isomerisation occurred on different types of site (i.e. 3M and 2M respectively) [25], where such sites have been defined [26]. Not only do such unsupported polymers show low selectivity with regard to hydrogenation over isomerisation, but they also deactivate. Thus the unsupported Rhdiisocyanobiphenyl polymers are quite stable to at least 469K in H 2 [27] but their meagre turnover frequencies in cyclohexene hydrogenation decreased rapidly with the use above 363K. It may be that while in the unsupported diisocyanobenzene polymers the benzene rings lie in the x-y plane and control the availability of the metal centre very little, in the supported polymer interactions with the support may cause the benzene tings to rotate and hinder access to the metals and induce some selectivity. Shape specificities of aromatic tings have been considered as one of the important factors governing the rate of hydrogenation of styrenebutadiene copolymers (AB & ABA block) using Wilkinson's catalyst [17] and the shape selectivity in the clathration of o-diborombenzene and o-dichlorobenzene over their meta- and para-isomers and catalysis of a two-dimensional square network material {[Cd(4,4'bipyridine) 2] (NO3)2}~ [28]. Figure 5 may hint at this with changes in conjugation at the benzene ring and changes in the CN bond strength. Here, additional work is required to increase selectivity by the introduction of alkyl groups at the 3-position on the aromatic ring to control the metal centres even more effectivitely (electronically and/or sterically)with Rh and with other transition metals, Thus
1092 one wonders what the steric control would be like in alkene hydrogenation when either (i) the alkene was larger or had a longer chain length or had bulky functional groups, or (ii) the diisocyano ligand contained a bulky side group (e.g. 2,4-diisocyanotoluene or a shorter bridge (e.g. 1,3-diisocyanopropene) [29]. These matrices can be applied to other (Group 1B and transition) metals and immobilized them upon a wide range of supports [30]. Alternatively, it may be that chemically-anchoring [31] will be valuable for incorporating an active molecular precursor (e.g. (MeO)3Si(CH2)2- 2-pyridyl..Rh) onto an oxide support. Catalyst preparations are said by some to be as much art as a science [32]. It is hoped that this approach allows more control over such critical area of catalysis. It may be that species once thought of as homogeneous catalysts (e.g. [15]) will eventually come to fruition in control of heterogeneous catalysts. 5. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of SERC (Grant No. GR~/58021) for MSWV. REFERENCES
.
3. .
"
6. .
.
9. 10. 11. 12. 13. 14. 15. 16.
J.H.A. Martens, R. Prins and D.C. Koningsberger, J. Phys. Chem. 93 (1989) 3179; R. Kieffer, A. Kiennemenn, M. Rodriguez, J.P. Hindermann and A. Deluzarche, Acta. Simp. Iberoam., Catal. 9th. 2 (1984) 1509; US patents 541,660, 4096164, 4101450, 4162262. D.E. Bergbreiter and R. Chandran, J. Amer. Chem. Sot., 109 (1987) 174. B. Bosnich (ed.) Nato ASI Series in Appld. Sci. 103E. 'Asymmetric Hydrogenation' (1986). M.M. Bhasin and G.L. O'Connor, Belgian Patent 824 (1975) 822; M.M. Bhasin, W.J. Bartley, P.C. Ellgen and T.P. Wilson, J. Catal. 54 (1978) 120; W. Liu etal., Sur. Sci. 180 (1987) 153; J.R. Shapely etal., Inorg. Chem. 21 (1982) 3295. J.B.F. Anderson, R. Burch and J.A. Cairns, J. Chem. Soc. Far. Trans I, 83 (1987) 913. P.A. Sermon, Y. Wang, M.S.W. Vong, Y. Sun and M.A.M. Luengo "Molecular Precursors and Probes for Designer Catalysts: The Quest for Complementarity" (1994). S.D. Jackson, R.B. Moyes, P.B. Wells and R. Whyman, J. Chem. Soc. Far. Trans I 83 (1987) 905; V.D. Alexiev, N. Binsted, J. Evans, G.N. Greaves and R.J. Proce, J. Chem. Soc. Chem. Comm. (1987) 396; M. Capka etal., RKCL, 31 (1986) 41; I.M. Saez etal, J. Chem. Soc. Chem. Comm. (1987) 361; P. Escaffre etal., J. Chem. Soc. Chem. Comm (1987) 146. K. Krogmann, Angew Chem. 8 (1969) 35. S.A. Lawrence, P.A. Sermon and I. Feinstein-Jaffe, J. Molec. Catal. 51 (1987) 117. I.L. Finar, Organic Chemistry, 6 Ed. Vol.1, Longman (1973), Ch. 20. S.A. Lawrence, K.A.K. Lott, P.A. Sermon. E.L. Short and I. Feinstein-Jaffe, Polyhedron 6 (1987) 2027. A.E. UnderhiU and D.M. Watkins, Chem. Soc. Rev., 9 (1980) 429; A.E. Underhill, Philo. Trans. Roy. Soc., 314A (1985) 125. K.R. Mann, N.S. Lewis, V.M. Miskowski, D.K. Erwin, G.S. Hammond and H.B. Gray, J. Amer. Chem. Soc. 99 (1977) 5525. A. Efraty and I. Feinstein-Jaffe, Inorg. Chem., 21 (1982) 3115; I' Feinstein-Jaffe and A. Efraty, J. Mol. Catal., 35 (1986) 285; 40 (1987) 1. C.O. Connor and G. Wilkinson, J.Chem. Soc. A (1968) 2665. K. K. Turisbekova, L.P. Shuikina, O.P. Parenago and V.M. Frolov, Kinetika i kataliz,
1093
17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.
29, (1988) 1023. X. Guo, .J. Scott and G.L. Rempel, J. Mol. Catal., 72 193 (1992); M. Carvalho, L.F. Wieserman and D.M. Hercules, Appl. Spectros. 36 (1982) 290. A. Efraty, I. Feinstein, F. Frolow and L. Wackerle, J. Am. Chem. Soc., 102 (1980) 6343. A. Efraty, I. Feinstein, L. Wackerle and A. Goldman, J. Org. Chem., 45 (1980) 4059. K.R. Mann, N.S. Lewis, R.M. Williams, H.B. Gray and J.G. Gordon, Inorg. Chem., 17 (1978) 828. A. Efraty, I. Feinstein, F. Frolow and A. Goldman, J. Chem. Soc. Chem. Comm., (1980) 864. P.N. Rylander, H. Greenfield and R.L. Augustine, Marcel Dekker, Catalysis of Organic Reactions (1988); J.P. Boitiaux, J. Cosyns and E. Robert, Appl. Catal., 35 (1987) 193. R.L. Banks and G.C. Bailey, I. Eng. Chem. Prod. Res. Dev., 3 (1964) 170. P.A. Sermon and G.C. Bond, J. Chem. Soc. Fara. Trans. I, 72 (1976) 745. R.L. Augustine, M.M. Thompson and M.A. Doran, J. Chem. Soc. Chem. Comm. (1987) 1173. S. Siefel, J Outlaw and J. Garti, J. Catal., 52 (1978) 102. H. Bonnemann, W. Brijorix, R. Brinkmann, E. Dinjus, R. Fretzen, T. JouBen and B. Korall, J. Mol. Catal., 74 (1992) 323. M. Fujita, Y.J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 116 (1994) 1151. F. Bonati and L. Malatesta, Isocyanide Complexes of Metals, Wiley, Interscience. P.A. Sermon, M.S.W. Vong and A. Jones, Preparation of supported metal catalysts via complexation with oxide-supported diisocyano networks, (in preparation). M. Capka, M. Czakoova, J. Hjortkjaer and U. Schubert, React. Kinet. Catal. Lett., 50 (1-2) (1993) 71. TechAlert, Dept. Trade & Industry, Chem. Britain, (1989) 1192.
1085
Preparation of Rh-Co/AI203 heterogeneous catalysts using a diisocyanoligand as an integral design component M.S.W. Vong and P.A. Sermon Solids and Surfaces Research Group, Department of Chemistry, Brunel University, Uxbridge, Middlesex UB8 3PH, UK Here consideration has been given to the potential for diisocyanide ligands to complex a transition metal (TM) cation to produce a monolayer of a highly-porous polymer in which more than 95% of metal atoms are constrained in well defined and characterisable active states with stable and known metal-metal coupling, symmetry and oxidation state. Catalysts prepared in this way have shown activities and selectivity in alkene hydrogenation, whereby the molecular network provides the shape-selectivity and the guest metal ion provides, the chemical reactivity and selectivity. This will probably lead to a new range of highly active and selective TM mono- and bi-metallic catalysts and adsorbents which could be of immense technical and scientific importance. 1. I N T R O D U C T I O N In recent years much attention has focused on the production of novel catalysts, adsorbents and sensors with high reactivity and selectivity9 Rh is a particularly interesting metal to consider in this context in that it is selective in alkene hydrogenation [1, 2], asymmetric hydrogenation [3] and CO/CO 2 hydrogenation [4]. In essence only a small fraction of Rh atoms in traditional supported catalysts are highly active [5] and their activity and selectivity are also affected by the fractal dimension of catalyst surfaces through the rates of reactants and products diffusion to and from the active sites [6]. Also immobilized and heterogenized transition metal complexes act as catalysts for homogeneous reactions and have become an area of intense catalytic interest. Others have used supported complexes, clusters [7] and polymers which are catalytically active. Molecular precursors may be one answer of particular relevance to highly dispersed Rh. The aim of this work was to use molecular receptors to produce, two- and three-dimensional networks into which a Rh reaction centre could be constrained so as to control their chemical nature and stereo environment to produce catalysts of high activity-selectivity in hydrogenation. In such an aproach to reactivity-control, bridging aryl-diisocyanide ligands (e.g. CNX-NC, where X is a benzene, diphenyl, etc group), are interesting in that (i) CN- groups chelate very effectively with metals via MdCxv)-to CN(2~) and CNfss* )to Md(x2_v2) overlap [8], producing net electron donation to Rh [9]. - (iO there is extended delocalisation in the x-y plane of the 'polymer' which is greatest when conjugation [10] is present and this produces substantial stability for the supported networks constraining the Rh +. (iii) metal-metal bonding is also allowable in the z-direction if multilayers are produced on a support (between the x-y laminae) since the sheets then stack up with metal centres aligned with weak dz2-dz2 a d Pz-Pz overlap. Thus in terms of metal-metal bonding and one-dimensional conductivity, ~ere are "similarities between the diisocyanocomplexes [11] and other one-dimensional conductors K2Pt(CN)4.Brx.3H20 [ 12].
1086 Diisocyano-Rh dimers photocatalytically decompose water [13] and diisocyano complexes catalyze hydrogenation and isomerisation of alkenes and alkynes (although they are far less active than the Wilkinsons catalyst [14]). Alkene hydrogenation is a probe reaction for such reaction centres, especially since the hydrogenation of alk-1-enes over Wilkinson's catalyst [hydrido-carbonyl tris(triphenylphosphine).Rh +] in benzene is quite selective (i.e. was 45 times faster than the cis-alk-2-ene [ 15]). Surprisingly, the active centres in such catalysts are still not entirely understood, despite extensive analysis [16]. Rh complexed with 4,4'-diisocyanobiphenyl and 1,4-diisocyanobenzene is active in hydrogenation and isomerization of 1-hexene [ 14], while Rh complexed with aliphatic amines is active in catalysis of hydrogenation of alkenes and cycloalkenes [16]. Ligands such as the 1,4-diisocyanobenzene or 1,4-phenyl-diisocyanide have been described as rigid collinear bridging ligands, which separate Rh ions by 1.2nm [ 17]. This has been used exclusively here and is denoted L (e.g. the unsupported polymer is given by [RhL2] n if we ignore any halogen and water components). The activity in hydrogenation of butenes of oxide-supported Rh/Co 1,4-diisocyanobenzene complexes is compared with that for simple impregnated catalysts. The unsupported Rh-diisocyanobiphenyl polymers are quite stable to 469K-617K in H 2 [9] but turnover frequencies in cyclohexene hydrogenation 293K were 3000 times lower than for silica-supported Rh and also decreased rapidly with use above 363K; hence the approach here to use the supported polymers. 2. EXPERIMENTAL 1,4-diisocyanobenzene (L) (Aldrich, 99% purity); [Rh(Co)2C1]2 (Aldrich; >97% purity), CoC12 (Fisons, >97% purity), A1203 (Degussa; 99% purity) and SiO 2 (Davison 923) were used.
2.1. Preparation 1,4-diisocyanobenzene undergoes a facile reaction with tetracarbonyl-bis-lxchloridedirhodium to produce a bulk tetragonal Rh-l,4-diisocyanobenzene chloride polymer with a [RhL2+.C1-.xH20] network. 2(CN-~-NC) + 1/2[Rh(CO)2C112
~. [Rh(CN-~-NC)2]+.CI'.x-H20 + 2CO
(1)
Here supported complexes were prepared by first impregnating the oxide with 5% w/w or 10% w/w ligand from a dichloromethane solution and then drying this under vacuum at room temperature. This was then allowed to exchange with a solution of [Rh(CO)2C1] 2 or a mixed solution of [Rh(CO)2C1]2 and CoC12 in ethanol (in which L was insoluble). From this the supported complex formed very quickly. After washing with fresh solvent (to remove unreacted chloride and ligand), the sample was then evacuated to dryness at room temperature.
2.2. Catalyst characterisation AA spectroscopy was used to study the rate and extent of metal incorporation under the above conditions. Powder X-ray diffractometry (Philip PWl710), FTIR (Perkin Elmer 1710), diffuse reflectance spectroscopy (Perkin Elmer Lamda 9 uv-vis near ir photospectrometer), XPS (Kratos ES300 with A1 radiation at 1468.6 ev), ESR (Varian E3 calibrated with DPPH) and TEM (Joel Instruments CX100) were used to confirm the formation of this ordered polymeric Rh + network on the oxide surface. TPR and in-situ FTIR were also used to study the thermal stability of the supported and unsupported complexes in H 2, CO and air. The total surface area of the oxide-supported catalysts was measured by BET N2 adsorption (Carlo Erba Sorptomatic 1800) at 77K.
1087
2.3. Catalytic activity measurements The activity of the catalysts in alkene hydrogenation was measured by passing a reactant stream (1.24kPa butene, cyclohexene or benzene, 76kPa H 2 and N 2 (balance to 101kPa)) through a catalyst sample (0.02-0.05 g) at 30 cm 3 rain "1 and 295-6K. Reaction products were analysed as a function of time by FID gas chromatography. Before reaction the sample were pretreated in hydrogen (393K) and then flushed with N 2 while cooling to the reaction temperature. 3. RESULTS
3.1. Characterisation AA showed that when the metal to ligand ratio (M/L) added was 0.5, 99% of the Rh and 80% of the Co was taken up rapidly by the A1203-supported diisocyanobenzene (compared to 20% and 10%, respectively without L) and there was also lower extents of leaching of A13+ from the support when L was present: Table 1 Metal concentrations in solution (ppm) measured by AA analysis .. L/Alumina
0 1 2 3 10
Alumin0t
[Rh] [Co] [AI]
[ma] [Co] [A1]
16.0 1.5 2.1 1.6 1.8
16.0 12.2 11.0 12.7 12.1
9.0 0.2 0.2 0.1 0.1
0.4 7.1 0.8 0.0 0.2
9.0 4.0 9.2 124.1 7.5 96.7 8.3 78.5 8.5 182.5
X-ray diffraction patterns obtained for the unsupported and oxide supported Rh complex corresponded to a crystalline network of tetragonal structure [14, 18] and FTIR spectroscopy indicated (see Figure 1) that the oxide supported complex is similar to the unsupported polymers [ 19] with absorption corresponding to the aromatic ring around 260 nm and the strong CN stretching frequencies at about 2120-2160 cm -1 being retained, although there is probably evidence of decrease conjugation in the oxide-supported state. Spectra of L/silica and L/alumina showed only a weaker CN absorption band indicating that the diisocyanobenzene ligand also complexed to a smaller extent with the A13+ or the Si 4+ ions on the surface of the catalyst supports. Interestly this absorption band was not observed for the 1,4-diisocyanobenzene ligand alone. The CN band of the diisocyano-network is relatively constant in oxidised or reduced atmosphere at 378K and when used in but-l-ene hydrogenation at 338K. UV-Vis diffuse reflectance spectra of the Rh/L polymer showed one intense absorption maximum at--400nm, close to that observed with Rh(CNPh)4BPh 4 monomer [20] and a board absorption band centred at 700nm-750nm, which is characteristic of weak intrachain Rh ....Rh interaction in polymers [18, 21] such as {Rh(diisocyanide)2+C1-]n. These electronic absorptions were asigned to the transition between energy levels in the A lz(dz2) bonding and A2u(p2p*) antibonding orbitals. The intensity of the above peaks decrease~l for
1088
d[H]/dt
% Transmission
b
(c) 3200
2400 1800 Wavenumber (cm -1)
Figure 1. FTIR spectroscopic properties of 3%Rh/0.6%Co/L/A120 3 (a), 10% L/A1203 (b) and A1203 (c). (*) denotes the CN stretching band of the diisocyano-network.
273
I
I
I
I
473
673
873
1073
T (K) Figure 2. TPR profile of 2%Rh/1.5%Co/ 10% L/A1203 (a), 2%Rh/L/A1203 (b) and unsupported Rh/L (c).
Figure 3. TEM micrograph of 4%Rh/L/A120 3 (x 893,000).
1273
1089 polymers in the supported state, which suggested a lower degree of metal-metal interaction (i.e. a decrease in the conjugation of the network and a resulting lower stability). The presence of Co in the complex increased the intensity of the absorption band and shifted it to longer wavbelength. Photoelectron spectroscopy revealed binding energies for Rh 3d5/2__(308.4eV) in the alumina-supported diisocyano network, which is well above the value (307.7eV) seen for Rh ~ The binding energies measured for Co 2pl/2 suggested that the Co was also in positive oxidation state. The constrained metal centres were stabilized by the extended conjugated ligand networks via electron delocalisation in the x-y plane. TG/DTA confirmed that Rh/Co/L/A120 3 was stable to at least 543K in H 2, with an exothermic reaction at only 581K. TPR profiles for the rhodium 1,4-diisocyanobenzene polymer are shown in Figure 2. The unsupported rhodium complex reduced at higher temperature (as much as 250K) than the Rh-Co/A120 3 impregnated catalyst. This was direct evidence of the stabilising effect of the polymeric network. The first reduction peak at low temperature was related to the liberation of chlorine ions as HC1 and the reduction of the rhodium centres (Rh:H = 1:4, after allowing for HC1 liberation). Hydrogen uptake at high temperature is believed to correspond to the degradation of the polymer matrix (L:H= 1:6). When the complex was supported on oxides, it became less stable and reduction occurred at lower temperature. Co incorporation increased the stability of the alumina-supported Rh network and raised the reduction temperature by 30K. The Rh-Co/L/A120 3 catalyst also showed an improved catalytic activity in alkene hydrogenation. Total surface areas of the catalysts were determined by BET N 2 adsorption at 77K and X-ray diffraction line broading (XRDLB). Table 2 Total surface areas (S) and average crystallite sizes (d) of the Rh complex catalysts* S (m2 g-l) AhO 3 (C) 100 L/A120 3 125 1%Rh/L/A120 3 144 4%Rh/L/A120 3 170 RhL 2 90 3 % Rh/0.575 % Co/L/A1203 109 2%Rh/1.15 %Co/L/A1203 107 (CoL was amorphous and showed no structure which was
d (nm)
47.9 43.3 41.2 14.9 detectable by XRD).
* deduced from BET and XRDLB at 7.8~ The Rh-1,4-diisocyanobenzene polymer was highly porous and revealed a Type II N 2 adsorption isotherm. It possessed an extensive rnesoporous structure and a zeolite-like microproous structure. The the total surface area of the non-porous A120 3 support was increased by L and metal-L complexation. Thus this increase in porosity suggested that these materials could be used as a selective adsorbents. Transmission electron micrograph (Figure 3) of the alumina supported Rh-diisocyano complex shows semicrystalline layer structure and unlike the impregnated catalyst, no Rh particles were observed.
3.2. Catalytic activity In Figure 4 the activity in hydrogenation of butenes of this alumina-supported 1,4diisocyanobenzene-constrained Rh/Co catalyst is shown and is compared to the characteristic
1090
nmol/g cat./min.
O
60
O
O
(a)
O 40 20 A
ID (b) 800 6OO
-
~
O-," I v
9
o
~
o
""
200 1
0
I
I
20
! 40
I
I
I
60
t (min)
Figure 4. Activities of 3%Rh/0.6%Co/I.JA120 3 (a) and 3%Rh/0.6%Co/A1203(b ) in hydrogenation of but-1-ene (O), cis-but-2-ene (e) and trans-but-2-ene (0) at 295K.
b
P
Y Figure 5. Manner of adsorption of cis-but-2-ene ( ~ ) on Rh ( ~ ) (constrained by diisocyanobenzcne in the x-y plane) along the z-axis where steric control depends upon alignment of aromatic rings and this will depend upon interactionswith the oxide support. The structuregiven is also thatseen for unsupported networks.
1091 of those of the corresponding impregnated materials. F~st the rates of butene hydrogenation over both catalysts are reasonabUy stable with reaction time. Second, unlike the impregnated catalyst, the supported polymer did shows stereoselectivity with decreasing rates in the sequence: but-1-ene > cis-but-2-ene > trans-but-2-ene even though the apparent hydrogenation activity of the supported polymer was 104-105 folds lower than the conventional Rh-Co/A12Oy For but-l-ene hydrogenation there was no significant isomerisation to but-2-ene isomers for the alumina-supported polymer, but only for the impregnated catalyst (as seen previously [22]). Others have found that the cis:trans ratio in isomerisation products accompanying alkene hydrogenation was very variable [23]. 4. DISCUSSION Often homogeneous catalysts are more selective (although more subject to diffusionlimitation) than heterogeneous ones [15]. Attempts to induce higher selectivity on metal active centres in heterogeneous catalysts are important in that the products would not be subject to the diffusion limitation to the extent that homogeneous catalysts are. In addition, immobilization of homogeneous catalysts using ligands would eliminate the difficulties in separating catalysts from reactants and/or products and allow catalysts to be regenerated and in some cases, reduce rate of observed catalyst deactivation. It appears that one mode of steric and/or electronic control is via a stabilising network to constrain the metal ions [22] using pre-adsorbed bridging aryl-diisocyano ligands which (i) chelate very effectively, (ii) have a stability enhanced by extended delocalisation and conjugation in the x-y plane, and (iii) allow metal-metal bonding in bulk networks in the zdirection (between the x-y laminae). I n the unsupported state such polymers do not appear to show this selectivity. At 298K and 2 atmos H 2 the Pd 0 complex of 4,4'diisocyanobiphenyl catalysed the hydrogenation of hex-1-ene to n-hexane [9, 14] but also produced trans- and cis-hex-2-ene in the ratio 4 (as did a 10%Pd/C catalyst) by isomerisation. However, the precise product ratio varied with time in a manner reminiscent of an alkene titration [24] with isomerisation at intermediate times. It may be that with such complexes the H concentration at Pd centres changes with the approach used on but-l-ene titration was to assume that hydrogenation and isomerisation occurred on different types of site (i.e. 3M and 2M respectively) [25], where such sites have been defined [26]. Not only do such unsupported polymers show low selectivity with regard to hydrogenation over isomerisation, but they also deactivate. Thus the unsupported Rhdiisocyanobiphenyl polymers are quite stable to at least 469K in H 2 [27] but their meagre turnover frequencies in cyclohexene hydrogenation decreased rapidly with the use above 363K. It may be that while in the unsupported diisocyanobenzene polymers the benzene rings lie in the x-y plane and control the availability of the metal centre very little, in the supported polymer interactions with the support may cause the benzene tings to rotate and hinder access to the metals and induce some selectivity. Shape specificities of aromatic tings have been considered as one of the important factors governing the rate of hydrogenation of styrenebutadiene copolymers (AB & ABA block) using Wilkinson's catalyst [17] and the shape selectivity in the clathration of o-diborombenzene and o-dichlorobenzene over their meta- and para-isomers and catalysis of a two-dimensional square network material {[Cd(4,4'bipyridine) 2] (NO3)2}~ [28]. Figure 5 may hint at this with changes in conjugation at the benzene ring and changes in the CN bond strength. Here, additional work is required to increase selectivity by the introduction of alkyl groups at the 3-position on the aromatic ring to control the metal centres even more effectivitely (electronically and/or sterically)with Rh and with other transition metals, Thus
1092 one wonders what the steric control would be like in alkene hydrogenation when either (i) the alkene was larger or had a longer chain length or had bulky functional groups, or (ii) the diisocyano ligand contained a bulky side group (e.g. 2,4-diisocyanotoluene or a shorter bridge (e.g. 1,3-diisocyanopropene) [29]. These matrices can be applied to other (Group 1B and transition) metals and immobilized them upon a wide range of supports [30]. Alternatively, it may be that chemically-anchoring [31] will be valuable for incorporating an active molecular precursor (e.g. (MeO)3Si(CH2)2- 2-pyridyl..Rh) onto an oxide support. Catalyst preparations are said by some to be as much art as a science [32]. It is hoped that this approach allows more control over such critical area of catalysis. It may be that species once thought of as homogeneous catalysts (e.g. [15]) will eventually come to fruition in control of heterogeneous catalysts. 5. ACKNOWLEDGEMENTS The authors gratefully acknowledge the support of SERC (Grant No. GR~/58021) for MSWV. REFERENCES
.
3. .
"
6. .
.
9. 10. 11. 12. 13. 14. 15. 16.
J.H.A. Martens, R. Prins and D.C. Koningsberger, J. Phys. Chem. 93 (1989) 3179; R. Kieffer, A. Kiennemenn, M. Rodriguez, J.P. Hindermann and A. Deluzarche, Acta. Simp. Iberoam., Catal. 9th. 2 (1984) 1509; US patents 541,660, 4096164, 4101450, 4162262. D.E. Bergbreiter and R. Chandran, J. Amer. Chem. Sot., 109 (1987) 174. B. Bosnich (ed.) Nato ASI Series in Appld. Sci. 103E. 'Asymmetric Hydrogenation' (1986). M.M. Bhasin and G.L. O'Connor, Belgian Patent 824 (1975) 822; M.M. Bhasin, W.J. Bartley, P.C. Ellgen and T.P. Wilson, J. Catal. 54 (1978) 120; W. Liu etal., Sur. Sci. 180 (1987) 153; J.R. Shapely etal., Inorg. Chem. 21 (1982) 3295. J.B.F. Anderson, R. Burch and J.A. Cairns, J. Chem. Soc. Far. Trans I, 83 (1987) 913. P.A. Sermon, Y. Wang, M.S.W. Vong, Y. Sun and M.A.M. Luengo "Molecular Precursors and Probes for Designer Catalysts: The Quest for Complementarity" (1994). S.D. Jackson, R.B. Moyes, P.B. Wells and R. Whyman, J. Chem. Soc. Far. Trans I 83 (1987) 905; V.D. Alexiev, N. Binsted, J. Evans, G.N. Greaves and R.J. Proce, J. Chem. Soc. Chem. Comm. (1987) 396; M. Capka etal., RKCL, 31 (1986) 41; I.M. Saez etal, J. Chem. Soc. Chem. Comm. (1987) 361; P. Escaffre etal., J. Chem. Soc. Chem. Comm (1987) 146. K. Krogmann, Angew Chem. 8 (1969) 35. S.A. Lawrence, P.A. Sermon and I. Feinstein-Jaffe, J. Molec. Catal. 51 (1987) 117. I.L. Finar, Organic Chemistry, 6 Ed. Vol.1, Longman (1973), Ch. 20. S.A. Lawrence, K.A.K. Lott, P.A. Sermon. E.L. Short and I. Feinstein-Jaffe, Polyhedron 6 (1987) 2027. A.E. UnderhiU and D.M. Watkins, Chem. Soc. Rev., 9 (1980) 429; A.E. Underhill, Philo. Trans. Roy. Soc., 314A (1985) 125. K.R. Mann, N.S. Lewis, V.M. Miskowski, D.K. Erwin, G.S. Hammond and H.B. Gray, J. Amer. Chem. Soc. 99 (1977) 5525. A. Efraty and I. Feinstein-Jaffe, Inorg. Chem., 21 (1982) 3115; I' Feinstein-Jaffe and A. Efraty, J. Mol. Catal., 35 (1986) 285; 40 (1987) 1. C.O. Connor and G. Wilkinson, J.Chem. Soc. A (1968) 2665. K. K. Turisbekova, L.P. Shuikina, O.P. Parenago and V.M. Frolov, Kinetika i kataliz,
1093
17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32.
29, (1988) 1023. X. Guo, .J. Scott and G.L. Rempel, J. Mol. Catal., 72 193 (1992); M. Carvalho, L.F. Wieserman and D.M. Hercules, Appl. Spectros. 36 (1982) 290. A. Efraty, I. Feinstein, F. Frolow and L. Wackerle, J. Am. Chem. Soc., 102 (1980) 6343. A. Efraty, I. Feinstein, L. Wackerle and A. Goldman, J. Org. Chem., 45 (1980) 4059. K.R. Mann, N.S. Lewis, R.M. Williams, H.B. Gray and J.G. Gordon, Inorg. Chem., 17 (1978) 828. A. Efraty, I. Feinstein, F. Frolow and A. Goldman, J. Chem. Soc. Chem. Comm., (1980) 864. P.N. Rylander, H. Greenfield and R.L. Augustine, Marcel Dekker, Catalysis of Organic Reactions (1988); J.P. Boitiaux, J. Cosyns and E. Robert, Appl. Catal., 35 (1987) 193. R.L. Banks and G.C. Bailey, I. Eng. Chem. Prod. Res. Dev., 3 (1964) 170. P.A. Sermon and G.C. Bond, J. Chem. Soc. Fara. Trans. I, 72 (1976) 745. R.L. Augustine, M.M. Thompson and M.A. Doran, J. Chem. Soc. Chem. Comm. (1987) 1173. S. Siefel, J Outlaw and J. Garti, J. Catal., 52 (1978) 102. H. Bonnemann, W. Brijorix, R. Brinkmann, E. Dinjus, R. Fretzen, T. JouBen and B. Korall, J. Mol. Catal., 74 (1992) 323. M. Fujita, Y.J. Kwon, S. Washizu and K. Ogura, J. Am. Chem. Soc., 116 (1994) 1151. F. Bonati and L. Malatesta, Isocyanide Complexes of Metals, Wiley, Interscience. P.A. Sermon, M.S.W. Vong and A. Jones, Preparation of supported metal catalysts via complexation with oxide-supported diisocyano networks, (in preparation). M. Capka, M. Czakoova, J. Hjortkjaer and U. Schubert, React. Kinet. Catal. Lett., 50 (1-2) (1993) 71. TechAlert, Dept. Trade & Industry, Chem. Britain, (1989) 1192.