Effects of loading on structure of Rh zeolite catalysts and their activity for methanol carbonylation

Effects of loading on slructure of Rh zeolite catalysts and their activity for methanol carbonylation S. Lars T. Andersson

Department of Chemical Technology, Lund Institute of Technology, PO Box 740, S-220 07 Lund, Sweden and Michael S. Scurrell*

Intituttet for Kemiindustri, Technical University of Denmark, DK-2800 Lyngby, Denmark (Received 19 November 1982; revised 2 February 1983) Rhodium X zeolites prepared from RhCI 3 and (Rh(NH3)sCI)CI 2 by ion exchange at room temperature (with Rh Ioadings from 0.38 to 4.0 wt%) have been studied with ESCA, volumetric CO-adsorption and measurements of activities for carbonylation of methanol. The zeolites are predominantly exchanged with RhCI(H20)~ + and Rh(NH3)sCI 2+, respectively. A surface enrichment in rhodium is seen, particularly at low metal Ioadings. A more homogeneous distribution at higher Ioadings is associated with a reduced CO/Rh ratio and a lower unit Rh activity in the carbonylation of alcohols. Rh(lll) species are present initially but lower valent species can be observed after thermal vacuum treatment of the samples. Samples with the highest unit rhodium activity (low rhodium loading) contain about 2 Rh atoms per unit cell in the surface layers and the CO/Rh ratios are just below 2.0, whereas samples with lower unit activity (higher rhodium loading) contain about 5-10 Rh atoms per unit cell in the surface layers and lower CO/Rh ratios. The Rh(NH3)sCI-NaX and RhCI3-NaX series exhibit somewhat different detailed behaviour. Keywords: Rh zeolite; methanol carbonylation; ESCA; X.p.s.; Rh loading; CO-adsorption

INTRODUCTION Rhodium zeolite catalysts have frequently been used in the carbonylation of alcohols 1-x2 and have also been the object of spectroscopic studies 12-21. A low temperature preparation (LTP) of a [Rh(NH3)sC1]Clz-NaX 9 zeolite has a much higher activity than an earlier studied high temperature preparation (HTP) of a RhC13-NaX zeolite 4' s Rates per unit of rhodium were almost the same as those found for catalysis by dissolved rhodium. Low temperature preparations of RhC13-NaY zeolites have similar high activities 1°' 11. Spectroscopic studies 9' 14, xs have revealed certain differences between the HTP RhC13-NaX and LTP [Rh(NH3)sC1]C12-NaX preparations. The former yields a lower dispersion of Rh. The pronounced difference in activity was considered to be partly expliquable on this basis but the manner in which Rh centres were located in the zeolite was also of importance. Data indicated that the RhC13-NaX sample contained more Rh at the external faces of the zeolite crystallites and the intracrystalline fields would not therefore be available to the extent they are in Rh(NH3)sC1-NaX where the * Present address: Chemical Engineering Research G r o u p - CSIR, PO Box 395, Pretoria 0001, Republic of South Africa. 0144-2449/83/030261-10503.00 © Butterworth & Co. (Publishers) Ltd.

evidence from ESCA suggests a more homogeneous distribution of the Rh within the zeolite framework. These findings prompted us to extend the studies to different ioadings ranging from 0.4 to 4 wt% Rh of both RhC13-NaX and Rh(NHa)sC1NaX samples all prepared at low temperature. A combination of catalytic and spectroscopic techniques have been used and in this contribution ESCA and volumetric CO-adsorption measurements are reported, and their relation to results obtained during catalytic studies pointed out. EXPERIMENTAL Catalyst preparation The rhodium zeolites were produced by ionexchange of NaX (Linde Division, Union Carbide, Batch B 882173) with aqueous solutions of the rhodium salts RhC13- 3H20 (Engelhard Industries) and [Rh(NH3)sC1]CI2 (prepared from RhCI 3 3H20(22)) at 25°C. Control experiments confirm that precipitation/deposition of hydrous rhodium oxides during ion-exchange is most unlikely to occur even with the use of RhC13, even for HTP (80°C) exchanged zeolites. Catalysts are designnated RhC13-NaX or Rh(NH3)sC1-NaX. The

ZEOLITES, 1983, Vo13,July 261

Rh zeolite catalysts: S. Lain T. Andersson and Michael S. Scurrell

amount of Rh incorporated in the zeolite was obtained from X-ray fluorescence analysis of the solution remaining after filtration of the zeolite suspension. The sodium displaced from the zeolites was measured by use of the same technique. Activity measurements The catalytic measurements carried out using previously described procedures 4' s are reported in detail elsewhere 23. Carbonylation of methanol was performed in a fixed-bed reactor and reaction rates obtained directly from the observed differential (< 10%) conve;rsions. CO, CH3I and CHaOH flow rates were 6.6, 0.017 and 0.25 #mol s -a respectively, and the total pressure was 1 bar. Catalyst samples (0.1-0.25 g) were pretreated by heating in flowing air, for 2 h. CO-adsorption measurements Volumetric CO adsorption measurements were performed on a micro-BET apparatus 24. Gas pressures were measured by use of a mercury manometer. Samples were degassed for 1 h at 100°C and 10-4Torr or for 4 h at 400°C followed by dead volume determination with N2 at 100°C. A second degassing for half the duration of the first one was performed before CO-adsorption at 100°C and 100 Torr for 1 h. ESCA studies Measurements were performed with an AEI ES200B electron spectrometer which has been described earlier 14 . An Al anode (1486.6 eV) was used and the analyser operated in the fixed retardation ratio mode. It was calibrated to give the following binding energy (B.E.) values for Au; 4f7/2 = 84.0 eV (FWHM = 1.8 eV), 4ds/2 = 335.2 eV, 4d3l 2 = 353.3 eV and 4p3 p = 546.4 eV in accordance with the most recent literature compilation 2s and also with the earlier calibration standards used here 26 for the Au 4fvp and 4ds/2 core lines. Charging effects were corrected for on selected samples with the Au-evaporation technique. The NaX zeolite B.E. thus derived were Ols = 531.5 eV, A12p -- 74.4 eV, Si2p = 102.3 eV and Nals = 1072.3 eV. These were used to check the analyser performance frequently and the maximum error in the measured B.E. is assumed to be + 0.2 eV. All samples were analysed at -- 100°C to minimize any interference from decomposition during analysis. Procedures adopted for quantitative analysis of spectral data are as used previouslyl4, is

measurements on pure samples of the salts RhC13. 3H20 and [Rh(NH3)sCI]C12. RESULTS AND DISCUSSION Location o f the rhodium centres In Figure 1 are shown the atom % Rh as measured by ESCA versus atom % Rh measured with X-ray fluorescence analysis (XRF) for some Rh-zeolites with varying loading. The latter values represent the actual loading of the catalysts (see Table 1 ) which agreed very well with the nominal values except for the nominal 1.3 wt% RhC13-NaX that was found to be 0.88 wt% Rh or 0.17 a t o m % Rh. The nominal 4 wt% RhC13-NaX was found to be 3.8 wt% Rh (0.756 atom % Rh). The broken line in Figure 1 represents the ideal result for a homogeneous distribution of the Rh-species. To determine what results might be expected if all Rh-species were present on the external surface of the zeolite particles a series consisting of ground physical mixtures of RhC13.3H20 and NaX was studied. As seen in Figure 1 the atom % Rh values obtained with ESCA are much higher than for the homogeneous case. However, the RhCI3.3H20 particles in that series are probably very large. If the salt was spread out in a uniform layer on the external surface of the zeolite particles, an even higher intensity could be expected. This was in fact observed for a HRh(CO)(PPh3)3-NaX catalyst, which was prepared by use of an impregnation procedure 27. This complex is too large in diameter to allow penetration into the zeolite cavities and all Rh must be confined to the external surface. However, it should be mentioned that the dis-

x

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Briefly, the peak areas above the base line were measured with a planimeter, and the figure was then corrected with the count rate setting and the particular elemental sensitivity factor. The latter were the same as used earlier 14' xs. The Rh, N and C1 elemental sensitivities were obtained from

262 ZEOLITES, 1983, Vol 3, July

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Alorn % Rh by XRF Figure 1 Atom % Rh measured by ESCA as a function of atom % Rh measured by X-ray fluorescense analysis (XRF) of some Rhodiun zeolites. (X) Ground mixture of RhCl 3 and NaX; (a) Rh(NH3)sCINaX. LTP; (o) RhCI~-NaX, LTP; (z~) RhCl3_NaX, HTP; (~) HRhCO(PPh~)3-NaX

Rh zeolite catalysts: S. L a r s T. Andersson and Michael S. Scurrell Table 1

Composition of Rh-zeolite catalysts

w/w% Rh Nominal

XRF

Formula

Average charge per Rh species

Plausible species exchanged

4a 1.3 a 0.4 a 4b 1.3 b 0.4 b

3.8 0.88 0.38

RhsNa77(AIO2)8+(SiO2)10~ Rh,.iNa~,H,.7 (AIO2)86(SiO=)~o+ Rho.sNas~gH2.~ (AIO2)86(SiO2)=o6

1.8 2.0 2.0

RhCI(H20)s=*, (Major species) R hCI (H 20) ~* ( Major species) RhCI(H20)I*, (Major species)

4 1.3 0.4

Rhs.2NaTs.s (AIO2)8+(SiO~) ~o+ Rh ~.~Na82.3Ho.3(AIO =)a6(SiO:) ,o6 Rho. s2NaBxoH 1.96(AIO2)8+(SiO2) ~o6

2.0 2.0 2.0

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0

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a Low temperature preparations (LTP) from RhCI~-3H20. b LTP from [Rh(NH3)sCI]CI 2. c High temperature preparations (HTP) from RhCI~. 3H=O.

persion which is not known, has a strong influence on the intensity 28. Using the model of Kerkhof et al. 29, the estimated intensity for a monolayer of RhC13.3H20 on the external surface of the zeolite leads to values of atom % Rh which are several times higher than the measured value for the HRh(CO)(PPh3)3-NaX catalyst. An early HTP (80°C) of RhC13-NaX 14 has an unexpectedly high Rh 3d intensity. Data points for this and the HRh(CO)(PPh3)3-NaX catalyst fall on the same line in Figure 1, and it therefore seems that in the HTP preparation most of the Rh present is located at the external surface. All LTP catalysts (25°C) give a higher atom % Rh by ESCA than would be expected for the case of a homogeneous distribution, but lower values than those obtained for the all surface Rh case. The series prepared from [Rh(NH3)sC1]C12 contain a more homogeneous distribution of rhodium than the RhC13-NaX series. Even more interesting is the fact that straight lines are not obtained, but instead curves with values of atom % Rh by ESCA falling off at high concentrations. There are two reasonable explanations; the most appealing is that the first Rh ions exchange in the surface (of the zeolite particles) which consequently becomes saturated at low loadings of Rh. At higher loadings the excess has to exchange in the bulk. The second alternative, that may operate simultaneously with the first, is that at higher loadings the dispersion of rhodium decreases, which in turn might imply that some direct deposition of rhodium species onto the external surface of the zeolite occurs. This, however, contradicts the results of sodium loss measurements which indicate incorporation of rhodium by a well behaved ion exchange. Of course the production of a well dispersed state of Rh during the vacuum treatment required for ESCA analysis cannot be discounted for these higher loaded samples. In Figure 2 atom % Na measured with ESCA versus the Rh loading measured with XRF are given.

Both the RhC13-NaX and the Rh(NH3)sC1-NaX series fall on approximately the same curves. The two samples with the highest loading yield a relatively large difference in atom % Rh but similar atom % Na by ESCA. It is concluded that the Rh(NH3)sCI-NaX series is ion-exchanged in a more well-behaved manner than the RhC13-NaX series, at least for the samples of highest loading. In Figure 3 the difference in atom % Na for the various Rh-zeolites and the pure zeolite -- all measured with ESCA -- are shown as a function of atom % Rh by ESCA. From this figure it is evident that the Rh(NH3)sC1-NaX series gives a slightly higher loss of Na. The curves obtained in Figures 2 and 3 indicate that the first Rh ions exchanged give the highest loss of Na by ESCA. A similar effect was reported 3° for Ag + in exchanged NaA zeolite. This effect may partly be explained by hydrolytic sodium loss, that was found to occur

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Atom % Rh by XRF Figure 2 Atom % Na measured by ESCA as a function of atom % Rh measured by X R F for some rhodium zeolites. (X) Ground mixture of RhCI 3 and NaX; (o) Rh(NH3)sCI-NaX ' LTP; (o) RhCI 3NaX, LTP; (zx) RhCI3-NaX, HTP

ZEOLITES, 1983, Vo13,July 263

Rh zeofite catalysts: S. Lars T. Andersson and Michael S. Scurrell I0

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Atom % Rh by ESCA NaXafter treatment with RhCl 3 and (Rh(NH~)sCl)CL" versus atom % Rh by ESCA. (o) Rh(NH~)~CI-NaX, LTP; (o) RhCl3-NaX , LTP; (z~) RhCl 3NaX, HTP. Dashed line represents the case o f ideal ion-exchange with 1 Rh ~: 2 Na F'igure3

D e c re a s e i n a t o m % ( [ N a ] N a x - [ N a ] R h x ) o f

5

o

°

o

o

shown in Figure 4. From the ratios it appears that the stoichiometry of RhNsC1 is obtained for all loadings in the Rh(NH3)sC1-NaX series whereas in the RhCI3-NaX series approximately RhC1 stoichiometry is obtained. The accuracy in the determination of the C1 concentration is poor due to the low intensity and high noise level; probably not better than + 30%. The very different behaviour of the HTP RhC13-NaX is seen in the very low CI/Rh ratio. A low ratio was also reported 19 for RhCI3-NaY prepared at 90°C. Evidently, the temperature of the aqueous RhC13 solution is very important. An increased temperature will increase the reaction rate of the ligand displacements which may be expected to be very slow in some cases 31. The Rh 3dsp binding energies reported in Figure 5 indicate that in all cases Rh is present in the + 3 valence state, and the N ls and C1 2p binding energies shown in Figure 6 are in reasonable agreement with data for the parent complexes for all samples. In conclusion, for all loadings [Rh(NH3)sC1] z+ species have exchanged for that series and for the RhC13-NaX series the predominant species exchanged is probably [RhCI(H20)s] 2+.

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F i g u r e 4 Atom ratio N/Rh and Cl/Rh measured by ESCA versus atom % Rh by XRF f o r s o m e Rh-zeolites. (o) Rh(NH3)sCI-NaX; (o) RhCI~-NaX; (t=) RhCI~-NaX, HTP

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for the samples of lower loadings. The m a x i m u m hydrolytic sodium loss measured was 0.3 atom % Na, which is almost negligible with the effects observed in Figures 2 and 3. The large deviation from the dashed line, which represents the case of ideal ion-exchange with 1 Rh --- 2 Na is most probably due to an exchange predominantly in the surface layers at low loadings mad an increased importance of exchange in the interior of the particles with increased loading. Considering the relatively low escape depth of the Na ls core line the appearance in Figure 3 seems reasonable. Data for the HTP RhC13-NaX sample is also shown in Figure 3, and the very different behaviour of that catalyst is clearly seen. It gives a much lower Na-loss, and if the Rh(NH3)sCI-NaX series is assumed to be ideally ion-exchanged, a simple comparison indicates that half the Rh species in the HTP catalyst have been incorporated as neutral Rh-species with no corresponding Na loss, or that Rh has exchanged as monovalent species. The N/Rh and CI/Rh atom ratios for both series are

264

ZEOLITE& 1983, Vol 3, July

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Figure5 Rh 3ds~ 2 binding energies (B.E.) and half w i d t h s ( F W H M ) versus atom % Rh f o r s o m e Rh-zeolites. (D) Rh(NH3)sCI-NaX; (o) RhCI~-NaX. Filled symbols represent parent complexes. (z~) RhCI~-NaX, HTP

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F i g u r e 6 N Is and CI 2p binding energies versus atom % Rh f o r s o m e Rh-zeolites. (o) Rh(NH3)~CI-NaX; (o) RhCI3-NaX. Filled s y m b o l s represent parent complexes

Rh

The HTP RhC13-NaX, however, has predominantly Rh(III)-species without any chloride, and considering the low sodium loss apparent from Figure 3 we would tentatively like to suggest that the major occurring species is [Rh(OH)2(H20)4])(see Table 1). Confirmation of this proposal is made complicated by the complex nature of RhC13 aqueous solutions in which a variety of species is expected to be present. Furthermore, aquation and hydrolysis may be accompanied by reduction and polymerization reactions that further complicate the situation 32.

zeolite catalysts: S. L a t ~ T. A n d e r s s o n a n d M i c h a e l S. S c u r r e l l

o E

Carbonylation activities The activities (given as turnover frequencies) for the catalysts of both series are shown in Figure 7. It is clear that the activity per Rh-unit falls with increased loading. Possibly the active sites are formed already at low loadings and that Rh in excess of that occupies what are, in the present context, catalytically inactive sites. Consideration of the ESCA leads to the suggestion that the active Rh-sites are formed in the external surface layer of the zeolite particles. We are not convinced that the intrusion of diffusion effects during catalytic carbonylation can alone explain this result 23. The initial configuration of these sites may be important for their catalytic properties. It is clearly seen in Figure 7 that the Rh(NH3)sC1-NaX series shows at all loadings a higher activity than is obtained for the RhC13-NaX series. Whether this is due to the presence of [Rh(NH3)sC1] 2+ species rather than [RhCI(H20)s] 2÷ species initially is uncertain since the ESCA data indicate quantitative differences as well. There may be differences in concentration gradients and dispersion that could explain the trend in activities. It is noteworthy that the HTP R.hC13-NaX catalyst with Rh confined to the

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~5

0 0

I

2

I 3

Z 4

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Figure 8 Rate of methanol carbonylation at 140°C versus ratio atom % Rh by ESCA to atom % Rh by XRF. Open symbols Rh(NH3)sCI-NaX. Filled symbols RhCI3-NaX. (o) = 0.4 wt%, (o) = 1.3 wt%, (~) = 4 wt%, nominal

extemal surface has an activity 50-100 times lower than for the LTP catalysts, but the fairly moderate difference in dispersion (vide infra) cannot alone, it is suggested, explain the large difference in activity. In Figure 8 the rate of carbonylation is given as a function of the ratio of surface to bulk Rh taken as atom % Rh by ESCA divided by atom % Rh by XRF. For both series the activity increases with the Rh surface/bulk ratio. The two curves look very similar, but with the difference that at each Rh surface/bulk ratio the Rh(NH3)sC1-NaX series catalysts give a higher reaction rate. The sharp increase in the reaction rate at the low loadings appears here to be connected to the increased surface to bulk ratio. The HTP sample of RhCI3NaX once again shows a typical behaviour with a rhodium surface/bulk ratio of 10 and a very low activity. The samples with the highest activity per Rh-unit contained by ESCA about 3 - 4 times as much Rh in the surface layer (see Figure 1 ), that is about 2 Rh-atoms per unit cell in the surface layer. Similarly, the samples of the highest loading contain about 5-20 Rh atoms per unit cell in the surface layer. The concept surface layer used here requires some explanation. The sampling depth of the ESCA analysis is not a well defined quantity. The intensity of a core line falls exponentially with the depth into the solid. The abruptness of the decrease is determined by the electron mean free

ZEOLITES, 1983, Vol 3, July 265

Rh zeolite catalysts: S. Lars T. Andersson and Michael S. Scurrell I0

initially - will be rehydrated when on stream. It seems much more plausible that these effects are directly due to changes in the Rh-centres, such as destruction of single centres and formation of agglomerates within the supercages and particles outside the crystallites. Since the effect of pretreatment temperature is larger for the samples of higher loading it seems likely that agglomeration is important. 0

0

~ 0

1 I00

i

•

IOta

I = I i 200 300 4u Pretreatment temperature (°C)

I 400

Figure 9 Rates for carbonylation of methanol at 140°C over some Rh-zeolites v e r s u s temperature of a~r pretreatment for 2 h. Rates given as ratios to the rate after 340 C pretreatment. (o) 0.4% R h ( N H a ) s C I - N a X ; (=) 4% R h ( N H a ) s C I - N a X

path. We have calculated this quantity as 2.5 nm for the Rh 3d lines in the zeolite 33. The outermost 2.5 nm layer will then contribute to two thirds of the full intensity for the infinitely thick layer. Since the unit cell is a cubic with 2.5 nm dimension 31 it follows that the main intensity comes from the first unit cell layer. If including two unit cell layers these will give almost 90% of the intensity for the indefinite thick layer. In order to explain the high activity of the catalysts with low loading one would suggest that the first 2 Rh-atoms per unit cell exchange in one specific site to a high degree on the average and at exchange levels in excess of that a more even distribution between sites is obtained. We would thus tend to assume that the catalytic activity is related to ions initially located in specific sites of the hydrated zeolite X. Besides the effect of changing the complex equilibria in the solution by exchanging at the high temperature, it may very well be that another distribution is obtained possibly involving less accessible sites. Monovalent and divalent ions are not expected to undergo exchange in an identical manner 34. In addition, dehydration of the zeolite, which is performed by heating in air flow at 340°C for 2 h prior to activity measurements, naturally, may change the positions of the cations. However, the zeolite will probably only be partially dehydrated at 340°C, and during catalysis rehydrated to some extent due to the formation of water in the reaction; 2CH3OH + CO ~ CH3COOCH3 + H20. Pretreatment at 340°C is not a necessary step in obtaining high catalytic activity. Indeed as seen in Figure 9, the reaction rates decrease with an increased temperature of pretreatment. One suggestion would be that this is connected with dehydration of the zeolite. However, the activities have been observed 23 to decrease also with reaction time during the first hour on stream. Since water is formed in the reaction, the zeolite -- if dehydrated

266

ZEOLITES, 1983, Vol 3, July

Changes in the rhodium-zeolites To get some insight into the changes in the Rhcentres during pretreatment and time on stream, ESCA studies on vacuum heat treated catalysts of both series were performed. Evidently, these treatments are not identical but are sufficiently indicative and furthermore are justified on the grounds that comparative activities result from degassing and calcination pretreatments. Figure 10 shows the percentage decrease in atom % Rh measured by ESCA due to heat treatment in vacuum at 400°C for 2 h. It is quite clear that for both series the decrease is greater for samples of higher loading. The decrease in intensity is interpreted as an agglomeration of the Rh-centres, and this will undeniably lead to a lower Rh 3d intensity. The 0.4% samples are apparently very stable in this respect and it is noteworthy that these catalysts show the highest intrinsic activity. Another explanation of the reduced Rh 3d intensity, albeit unlikely, is that upon reduction the Rh ions may migrate towards the interior of the zeolite particles, and thus escape detection by ESCA. Any such migration would probably be over a very limited distance and the cations would consequently still be within the sampling region. In Figure 11 the percentage decrease in atom % N and C1 are shown. The same trend as observed for the decrease in % Rh are obtained. However, the reduction in % N and % C1 is caused by the

40

30 u.I o:

o~

20

0

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I 0.2

i

I i I 0.4 0.6 Atom % Rh by XRF

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Figure 10 Percent decrease in atom % Rh ([Rh]_~oo-[Rh]4oo)/ [Rh]_~o o measured by ESCA after vacuum heat treatment at 400°C f o r 2 h of some Rh-zeolites. (o) R h ( N H 3 ) s C I - N a X ; (o) R h C I 3 - N a X ; (zx) RhCla_NaX, HTP

Rh zeofite catalysts: S. Lars T. Andersson and Michael S. Scurrell

these explains the residual C] present as shown in 80

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F i g u r e 11 Percent decrease in atom % N and CI measured by ESCA a f t e r v a c u u m h e a t treatment at 400°C for 2 h of some ah-zeolites. (=) N for Rh(NH3)sCI-NaX; (o) CI for Rh(NH3)sCI-NaX; (o) CI f o r

RhCI3-NaX; (&) CI for RhCI3-NaX, HTP

dissociation of NH3 and C1- ligands. It is seen from these data that the 0.4% samples actually undergo decomposition to some extent: 6% for the Rh(NH3)sC1-NaX catalyst and 17% for the RhC13NaX catalyst. Here possible complications arising from agglomeration were absent. A prolongation of the vacuum heat treatment will naturally lead to a higher degree of decomposition and a reduction in dispersion, but the rank of stability has clearly been shown. After prolonged treatments we have measured stoichiometries as low as RhC10.3 for the RhC13-NaX series and RhClo..N x for the Rh(NH3) sC1-NaX series whereas the HTP RhCla-NaX catalyst gives RhC10.1. Rh exchanged Y zeolites prepared from RhC1 a at high temperature and more sensitive to vacuum heat treatmcnt x9 and furthermore it was found that the Rh-NaY sample of low loading (18% exchange) was more stable than the high load samples (50% exchange) which agrees with our findings for the NaX zeolite supported Rh-species. In Figure 12 the Rh 3ds/2 B.E. for the catalysts after various heat treatments are shown. It should first be noted that the HTP RhCla-NaX catalyst which has been studied in detail earlier 14 does not give any change in the Rh 3ds/2 B.E. even after 15 h in vacuum at 200°C in contrast to the LTP RhC13-NaX catalysts that show a slight decrease in the Rh 3dsp B.E. The higher loaded samples seem to be slightly more easily reduced. The difference between the HTP and LTP RhC13-NaX catalysts underlines their different chemical nature as discussed above. A more severe vacuum heat treatment at 400°C for more than 2 h produces a Rh 3dsp B.E. at peak m a x i m u m in the range 307.0-307.2 eV, signifying formation of Rh metal 14. However, the broad, and in some cases asymmetric, nature of the Rh 3d core lines indicates the presence of substantial amounts of Rh(I) and probably some Rh(III). The presence of

Figure 11. Heating the catalysts of the Rh(NH3)sC1NaX series in vacuum at 400°C for 2 h produces Rh-species with a Rh 3dsp B.E. at peak m a x i m u m of about 308.2 eV, which is characteristic for many Rh(I)-complexes 3s. Peak shapes indicate, however, especially for lower loadings the presence of Rh(III) and the additional presence of Rh(0) cannot be excluded. It was shown in Figures 10 and 11 that the samples of lower loading showed a smaller decrease in the Rh, N and C1 atom %, with hardly any effect at all for the 0.4% catalyst. This could be explained by analogy with the thermal decomposition of Pt(NH3)6CI 4 and Pt(NH3)6-Y in He a6, and for the Rh-zeolites give the followhag decomposition products: Rh(NH3) s - nCll - m (Ozeol),, + m -- NaX, NH4X, NH4C1, HC1, NH3, Nz and HX. The complete absence of i.r. stretching vibrations of NH3 after decomposition of Rh(NHa)sC1-NaY in vacuum at 300°C 37 may be explained by the different nature of the NaX and NaY zeolites. A comparison of the N ls and C1 2p B.E. for the as received catalysts in

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Figure 12 Rh 3ds~2 binding energies of some Rh-zeolites a f t e r various heat treatments in vacuum. (D) Rh(NH3)sCI-NaX , 400°C, vac. 2 h; (o) RhCI3-NaX ' LTP, 200°C, vac. 15 h; (z~) RhCI3_NaX, HTP, 200°C, vac. 15 h; (&) RhCI3-NaX, HTP, 400°C, vac. 2 h; (e) RhCI~-NaX, LTP, 400°C, vac. 2 h

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F i g u r e 13 CI 2p and N Is binding energies for some R h - z e o l i t e s a f t e r v a r i o u s h e a t t r e a t m e n t s . (=) N Is-Rh(NH3)sCI-NaX, 400°C,

vac, 2 h; (o) CI 2p-Rh(NH 3)sCI-NaX, 400°C, vac, 2 h; (o) CI 2pRhCI3-NaX , LTP, 200°C, vac. 15 h; (=) CI 2p-RhCI3-NaX, LTP, 400°C, vac. 2 h; (z~) CI 2p-RhCI3-NaX, HTP, 400°C, vac. 2 h

ZEOLITES, 1983, Vo13,July 267

Rh zeolite catalysts: S. Lars T. Andersson and Michael S. Scurrell

Figure 6 and after vacuum heat treatment in Figure 13 reveals no dramatic effects. The average N ls B.E. seems to be slightly lower after heat treatment and furthermore the accuracy in the C1 2p B.E. determinations is not very high at the lower concentrations.

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Volumetric adsorption measurements

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The measurements of the volumetric adsorption of CO were performed with the objective of getting more direct information about the dispersion of the Rh-species." The results are presented in Figure 14. The dotted line shows the values expected for a CO/Rh stoichiometry of 2. The actual uptake at the conditions specified in Figure 14 was used instead of an extrapolated monolayer coverage at zero pressure, and it includes both reversibly and irreversibly adsorbed CO. The justification is that the adsorption at 100°C and 100 Torr lies in a plateau region of the isotherm as reflected in infrared spectroscopic studies 3s, and the coverage obtained will be a good approximation to the monolayer situation. The CO uptake on the zeolite itself was found to be negligible. The CO uptakes presented in Figure 14 are seen to increase with the metal loading of the zeolite. However, the increase is not linearly dependent on the loading, which indicates that the dispersion or rather the capacity for CO uptake per Rh unit is, in general, decreasing with increasing loading. The derived CO/Rh ratio are depicted in Figure 15 and here it is apparent that the ratio is generally higher for the Rh(NH3)sCI-NaX series. Heating at 400°C instead of 100°C during pretreatment produces a strong decrease in the CO/Rh ratio and the magnitude of the effect is greater for the Rh(NHa)sC1-

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Atom % Rh by XRF -l) Figure 14 Uptake of CO(cm3(NTP) gcat measured by volumetric CO-adsorption on some Rh-zeolites. (o) Rh(N H3)sCI-NaX; (o) RhCI3-NaX. Open symbols are degassing for 1 h at 100°C and l O - " T o r r followed by CO-adsorption for 1 h at 100°C and 100 Torr. Solid symbols are degassing for 6 h at 400°C and 10-" Torr followed by CO-adsorption for 1 h at 100°C and 100 Tort

268 ZEOLITES, 1983, Vol 3, July

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NaX samples. It is quite clear that this series exhibits a much greater sensitivity towards thermal treatment as was also shown above. A comparison with activity data in Figure 7 reveals that there is no correlation between activity and CO uptake. Figure 16, however, emphasizes that in general high CO/Rh ratios are associated with high unit Rh activities. The Figure also illustrates the contrasting behaviour of the Rh(NH3)sCI-NaX and RhC13-NaX series in that the greater sensitivity of the CO/Rh ratio to the rhodium loading of the former series for highly loaded samples is clearly seen from the relative shapes of the curves in the region of the origin. The anomalous behaviour of the low loaded RhC13-NaX sample is also apparent. This catalyst gives a CO/Rh ratio which is almost as low as that exhibited by the sample with the highest loading (Figure 15). The highest CO/Rh ratio is obtained at an intermediate loading but is still significantly below 2. It is important to emphasize that the vacuum heat treatment will give a partial decomposition of the Rh species initially present and in addition a loss of dispersion via agglomeration as shown by the ESCA studies. Consequently, for all heat treated samples several rhodium species such as Rh(III), Rh(I) and Rh(0) will be present. In addition, some Rh crystallites may also be formed. Since all these species can interact with CO with various stoichiometries, the overall CO/Rh ratio cannot give any detailed information on the relative abundance of the individual species. Moreover for metallic particles one can expect that the stoichiometry is particle size dependent 39. Overall, a high CO/Rh ratio approaching 2 suggests a relatively high abundance of Rh(CO)2 units, but for ratios less than 2 the fraction of Rh(CO)2 units present cannot be estimated on the basis of adsorption data alone. Some help is, however, provided by recent infrared spectroscopic studies on Rh(NH3)sC1-NaX catalysts 3s which demonstrate that mildly pretreated samples interact with CO to give Rh[(CO)2

Rh zeolite catalysts: S. Lars T. Andersson and Michael S. Seurrefl

CONCLUSIONS

a

The specific activities of rhodium-zeolites in the carbonylation of methanol appear to be related to the location of rhodium within the support matrix, this being in turn influenced by the preparation conditions, the source of rhodium and the nature of any pretreatments applied to the samples. In particular, high unit rhodium activities are associated with low loadings of rhodium and the preferred incorporation of rhodium ions in the surface layers of the zeolite up to an optimum level of occupancy of about 2 Rh ions per unit cell. At higher loadings a more homogeneous distribution of rhodium together with a higher occupancy of the nearsurface supercages are both associated with a marked lowering of the specific carbonylation activity. Additionally, an increased tendency for rhodium-rhodium interactions to occur is noted and agglomeration of rhodium under thermal conditioning probably leads to the formation of some metal crystallites. Rh(NHs)sCI-NaX and RhC1 sNaX samples are not equivalent as far as their detailed behaviour regarding rhodium location, CO adsorption and carbonylation activity is concerned. High temperature preparations based on RhC1 s contain a large excess of rhodium in the near-surface layers but exhibit low catalytic activity emphasizing that the optimum occupancy can easily be exceeded and/or that the nature of the exchanging cation is of significance.

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rco(lO-Smoi s-t gRh-L) Figure 16 CO/Rh mole ratios compared with rate of carbonylation of methanol at 140°C for rhodium zeolites pretreated at 340°C for 2 h with air flow. CO/Rh ratios measured as in Figure 14. (A) Samples pretreated at 400°C and 10 -4 for 6 h in total. (B) Samples pretreated at 100°C and 10 -4 T o r t for 1.5 h in total. (o) Rh(NH3)sCI-NaX; (e) RhCls-NaX

and Rhlll(co) units. Experiments with labelled CO have revealed that the former undergo exchange with lZCO at a rapid rate at 50°C but that the Rhm(CO) species are not reactive. Adsorption of 12CO on an outgassed 1.3 wt% Rh(NH3)sCI-NaX catalyst followed by evacuation and exposure to 13CO led to a x2COflaCO ratio in the gas phase which was consistent with the presence in the sample of Rhl(Co)2 and Rhm(CO) units in the relative abundance 12/88. This ratio would be expected to lead to a CO/Rh ratio of 1.1, identical to that obtained after adsorption at 100°C

(Figure 15). An additional complication may arise if Rh ions migrate into sites which are inaccessible to CO the hexagonal prisms or sodalite units 4°' 41 _ though we have no evidence for such behaviour in these samples.

In general high CO/Rh ratios obtained in adsorption experiments can be linked with high unit rhodium activities and a special role for Rh(CO)2 units is suggested. Reduced CO/Rh ratios are likely to arise in several ways including increased stabilization of Rhm(CO) units, increased incorporation of rhodium into inaccessible sites and formation of metallic crystallites. ACKNOWLEDGEMENTS Prof. S-T. Lundin is acknowledged for his generous support to this work. We are also indebted to Mr. S. Kiuru for assistance in the adsorption measurements and to Mr. J. Raskmark for carrying out the exchange reactions with 12CO and tsCO. REFERENCES 1

2 3 4 5

Nefedov, B. K., Sergeeva, N. S., Zueva, T. V., Shutkina, E. M. and Eidus, Ya. T. Izv. Akad. Nauk SSSR, Ser. Khim. 1976,

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Rh zeolite catalysts: S. Lars T. Andersson and Michael S. Scurrell

6 7 8 9 10 11 12

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