palladium-based catalysts supported on oligomeric aramides. a tpr investigation

JOURNAL OF

MOLECULAR CATALYSIS

Journal of Molecular Catalysis 94 ( 1994) 203-212

Palladium-based catalysts supported on oligomeric aramides. A TPR investigation

Francesco Arena, Giampietro Cum *, Raffaele Gallo, Adolf0 Parmaliana

Dipartimento di Chimica Industriale, Universid degli Studi di Messinn, Salita Sperone 31 (c.p. 29), I-98166

S. Agata di Messina, Italy

Received 2 I January 1994; accepted 11 July 1994

Abstract

The reduction pattern of PdCl, supported on oligomeric aramides has been investigated by temperature programmed reduction (TPR) measurements in the range of temperature - 80 to 305°C. The influence both of chemical composition of the oligomeric matrix and Pd loading has been ascertained. A peculiar metal-support interaction greatly affects the reducibility of Pd, allowing its stabilisation in the oxidised form (i.e., Pd* + ).

1. Introduction

A successful method to realize efficient catalytic systems is to immobilize transition metal derivatives on organic matrices. Thus, an improved reactivity and/or selectivity of the catalytically active species can be often obtained, as well as a simplified way to their recovery and recycling [ 11. For instance, Tang and Sher- rington stressed the importance to support PdCIZ precursor on organic polymers to stabilise Pd in an oxidised form so to allow an extension of the Wacker reaction to propene and other terminal alkenes; in this context they also reported a significant matrix effect on the reactivity of supported PdCIZ [ 21. In most cases the investigation of such heterogenized metal complexes, acting as catalysts, are pursued looking for similarities with their homogeneous counterparts; it seems, however, more rational to use an approach which accounts for their behaviour as if they were acting as heterogeneous catalysts [ 31.

* Corresponding author.

0304-5102/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDIO304-5102(94)00142-l

204 Francesco Arena et al. /Journal of Molecular Catalysis 94 (1994) 203-212

Several attempts to realize heterogenized catalytic systems suffered from some drawbacks, mainly due to the poor chemical, mechanical and thermal properties of the polymeric materials selected [ 41. This led to many discouraging results as far as the application to large scale industrial processes is concerned. Therefore, in an attempt to overcome the limitations described above, some palladium derivatives were made to interact with the existing chain terminal groups of an organic mac- romolecule, i.e. the oligo-p-phenylenterephthalamide, (OPTA) 1, to realize a system with peculiar catalytic and technological properties [5,6]. Noteworthy, our preparative procedure is simpler than in the case of most reported polymer-supported catalysts, in that neither functionalisation of the polymeric backbone is needed nor a specific ligand has to be linked as a preliminary step to the metal insertion to give a coordination complex [ 71. Besides, the relatively low average molecular mass ( = 800 a.m.u.) of the proposed carrier 1 allows for a high concen- tration of chain terminal groups to support transition metal ions. The unusual catalytic properties of Pd/OPTA system have been already successfully tested on some model reactions, such as hydrogenation and oxidation of carbon+arbon and carbon-oxygen multiple bonds [ 8,9] ; usually, high activity, selectivity and speci- ficity have been achieved. Previous work has been aimed to clarify the nature of the interaction between the metal and the oligomeric species and to elucidate the chemical structure of the title catalysts, together with a characterization of their physical and morphological properties [ 61.

The present paper reports preliminary results on TPR characterization of the title system and it is aimed to shed light on the behaviour of this catalytic system in model reactions where dynamic interactions between Pd precursor and organic matrix 1 (OPTA) occur. For reference, other similar supporting oligomeric species, such as 2 and 3 (see below), were also prepared and characterized.

2. Experimental

2.1. Materials

All reagents and solvents were analytical-grade quality and were used after re- crystallisation or distillation under reduced pressure.

(i) The oligomeric species 1 was prepared by interfacial condensation from an aqueous solution of 1 ,Cphenylenediamine dihydrochloride and a solution of terephthaloylchloride in cyclohexanone, according to the procedure previously reported

[WI.

H-[-NH-&H,-NH-CO-C,&---CO-],-OH (1)

(ii) Oligomer 2 was obtained by adding dropwise a chloroform solution of 1,4- cyclohexane-dicarboxylic acid chloride to a solution of 1,4-cyclohexanediamine in the same solvent, the molar ratio being 1: 1.4

Francesco Arena et al. /Journal of Molecular Catalysis 94 (1994) 203-212 205

H-[-NH-C6H1,,-NH-CO-C6H10-CO-_3.-OH (2)

(iii) Oligomer 3 was obtained as above by adding dropwise a cyclohexanone solution of terephthaloylchloride to an aqueous solution of 1,4-cyclohexanediamine in the molar ratio 1: 1.4.

H-[-NH-C6H1,,-NH-CO-CJ-I,+-CO-_I,--OH (3)

The solid materials obtained were filtered under reduced pressure, washed with hot water and acetone, then dried at 80°C under reduced pressure.

Palladium was added to the supports (1, 2 and 3) by wet impregnation with a water:acetic acid solution (9: 1) of PdClz [ 61. The Pd content of the catalysts was determined by thermogravimetry [ 101.

Temperature programmed reduction (TPR) runs were performed from - 80°C to 305°C at a heating rate of 12°C * min- ‘, using a 5.7% Hz/N2 mixture flowing at 30 mlemin-’ SIP. Further, an isothermal reduction lasting 5 min was performed at 305°C. The quantitative calibration of the Hz consumption peaks was made by monitoring the reduction of known amounts of PdClz (2.0-5.0 X 10e3 g) [ 111.

3. Results

The TPR profiles of (a) the bare support 1, (b) the bulk PdC& and (c) of the ‘as received’ 1.0% Pd/l catalyst, in the range - 80 to 305’C, are comparatively shown in Fig. 1.

Bulk PdClz (Fig. lb) undergoes reduction to Pd” in a single step, giving rise to a sharp and well resolved TPR peak with maximum (TM) at 79°C whilst the bare support 1 does not provide any evidence of reduction process in the investigated range of temperature (Fig. la).

Noticeable changes in the TPR behaviour of either PdCl* or carrier 1 are observed further to impregnation of the oligomeric matrix with the PdClz precursor (Fig. lc) . Indeed, such a pattern shows a small reduction peak with TM, = - 5°C whose intensity corresponds only to a small percentage ( = 5%) of the stoichiometric Pd2+ + Pd” reduction process; a broad peak is also observed at high temperature ( > 2OO”C), corresponding to an apparent H2 consumption of 970 pmol/g,,,, much larger than that expected on the basis of the whole reduction of the Pd2+ in the system (94 pmol/g,,,). In order to shed light on the unique TPR pattern of this 1 .O% Pd/ 1 catalyst, the effect of the metal loading as well as the influence of the chemical composition of the oligomeric matrix on the reducibility of supported PdC12 have been carefully examined.


I 1 I 1 1 I I

-25 85 195 305

T (“Cl

Fig. 1. TPR profiles of (a) ‘bulk’ support 1, (b) ‘bulk’ PdC& and (c) 1% Pd/l catalyst.

3.1. ESfect of Pd loading on the TPR projile of P&l catalysts

In Fig. 2 the TPR spectra of the (a) 0.5, (b) 1.0, (c) 1.5, (d) 4.3 and (e) 8.2% Pd catalysts supported on 1 are reported for comparison, whilst the temperatures corresponding to peak maxima (TM) are summarized in Table 1 along with the amount of Hz consumption for each system.

All these systems exhibit a similar pattern, featuring the presence of a small reduction peak, with TM, ranging between - 14 and - SC, along with a broad and convoluted Hz consumption which starts at T (TO) becoming progressively higher as the Pd loading decreases (Table 1) . Furthermore, on increasing the Pd loading we observe that: - other peak maxima appear at progressively lower T (Table 1) ; - the temperature of the above peak maxima shifts on the whole to lower T

(Table 1);


I

I I I I I I

-25 85 195 305

T (“Cl

Fig. 2. TPR profiles of (a) 0.5%. (b) 1.0%. (c) 1.5%. (d) 4.3% and (e) 8.2% Pd supported on matrix 1.

Table 1 TPR of differently loaded Pd/l catalysts

Sample Temperature (“C) Experimental Theoretical

Hz consumption H2 consumption

TO TM, T MZ Thl, T M4 (vol. GJ ) ( cc.mol. ~2;: )

1 _ _ 0 0

PdClz 20 - 19 - - 5631 5637

OS%Pd/l 151 -5 - _ 302 440 47

1 .O%Pd/l 122 -6 - - 300 970 94

1.5%Pd/l 102 -8 - 174 303 2100 141

4.3%Pd/l 64 - 14 _ 177 302 5680 404

8.2%Pd/l 37 - 13 107 165 300 7950 761

- the intensity of the overall Hz consumption is in any case more than ten times larger than that expected on the basis of the stoichiometric reduction of the Pd2 + content in the system investigated (Table 1) .


(P IP C

10 ,2: ‘-e 3

,,,,,,:

-25 85 195 305

T(t)

-

!

Fig. 3. TPR profiles of (a) 4.3% Pd/l, (b) 4.7% Pd/2 and (c) 5.3% Pd/3 catalysts.

3.2. TPR of PdCl, supported on carriers I, 2 and 3

The TPR profiles of (a) 4.3% Pd/l, (b) 4.7% Pd/2 and (c) 5.3% Pd/3 catalysts are compared in Fig. 3. Moreover, as previously observed for the bare support 1, both 2 and 3 matrices do not provide any evidence of Hz consumption or release in the Trange investigated (Table 2). The TPR spectra of PdC12 supported on different carriers (Fig. 3) displays some different features, as a consequence of the different chemical composition of carriers. Namely, the 4.7% Pd/2 system (Fig. 3b) shows both a small reduction peak with TM, = - 10°C whose intensity is comparable with that of the 4.3% Pd/l catalyst (Fig. 3a), and a broad and asymmetric H2 consumption band, with a main maximum at 192”C, which likely indicates the contribution of at least two convoluted peaks. The overall intensity of this H2 consumption peak equals to 375 pmol/g,,,, corresponding to = 85% of the H2 consumption


Table 2

TPR of PdCl, supported on different polymeric matrices

Sample Temperature (“C) Experimental Theoretical

Hz consumption H, consumption

TM T M? TM3 T M4 Td ( mm01 . g,i ) (mmol.g;i,)

1 _ _ 0 0 2 0 0 3 _ 0 0 4.3%Pd/l -14 - 177 301 - 5680 404

4.7%Pd/2 -10 - 192 - 298 375 ( -2290) a 440

5.3%Pd/3 -15 - 143 - _ 425 (-845) a 500

a Negative values reported in parentheses refer to the H, release process.

related to the stoichiometric reduction of the Pd2+ of the system (Table 2). Further, a steep drop in the baseline, giving rise to a negative peak, is related to a noticeable H2 desorption process characterized by a maximum desorption rate (r,) at 298°C. The deconvolution of these opposite signals shows a partial overlapping (dashed area) between H2 consumption and H2 release processes (Fig. 3b) and results in a H2 consumption of 420 pmol/g,,, and in a H2 release of 2290 pmol/g,,,.

The reduction pattern of the 5.3% Pd/3 catalyst (Fig. 3c) looks similar to that of the 4.7% Pd/2 catalyst, as we observe a main and broad H2 consumption band with TM2 = 145°C strictly convoluted with a negative peak, whose shape however does not feature a resolved maximum desorption rate in the temperature range 270- 305°C. Even in this case the H2 consumption corresponds to = 85% of the reduction of Pd2+ (Table 2)) whilst the intensity of the H2 desorption process is equal to 845

pmol/g,,,, which is about one third of that experienced for the corresponding process of the 4.7% Pd/2 supported system.

4. Discussion

Previous studies on the reactivity of the Pd/ 1 catalysts [ 8,9] provided evidence of a singular catalytic behaviour, probably related to the peculiar electronic properties of the active metal in such unconventional supports [6]. To gain further insights into these catalytic systems, we investigated the influence of the oligomeric support on the reducibility of the palladium derivative and its role in a hydrogenation+lehydrogenation process, which seems to affect the stabilization of the transition metal in its oxidized form, i.e., Pd2+ .

4.1. InfIuence of the support on the reducibility of PdCl,

The TPR spectra of the bare support 1 (Fig. la), of the ‘bulk’ PdC12 (Fig. lb) and of the 1% Pd/l catalyst (Fig. lc) clearly indicate that an unusual metal-


support interaction strongly inhibits the reducibility of the Pd precursor. In fact, a noticeable shift to high values both of the onset temperature (To) of the reduction process and of the peak maxima has been observed (Table 1). Indeed, if we disregard the peak at low T ( TM1 ), which represents a very small percentage of the Pd2+ content of the system and probably arises from the reduction of more reducible PdO species [ 111, we notice that on the whole the system exhibits a reactivity towards H, much lower than that experienced for conventional Pd2+ supported catalysts [ 11,121. To validate the above findings, in the hydrogenation process involving the phenylacetylene + styrene + ethylbenzene sequence, a maximum in the activity has been obtained with a Pd/l catalyst activated at high T (130°C)

[131. Furthermore, taking into account that in our case the carrier is able to interact

with PdC12 only by the chain terminal R-NH2 groups [ 61, then we can infer that the formation of rather stable PdC12. ( NH2-R)2 macrocomplexes renders Pd2+ hardly reducible. This hypothesis is well sustained by the fact that even for highly loaded Pd/l catalysts (Table 1) as well as for different chemical compositions of the matrices (Table 2), both temperatures related to the first maximum reduction rate of Pd2+ , i.e., T,, and TM3 are quite higher than those found for the reduction of the bulk PdC12 (Table 1) .

Also the considerable broadness of the H2 consumption peak associated with the reduction of Pd2+ is accompanied by the appearance of several reduction maxima (TM) in the TPR profiles of differently loaded Pd/ 1 catalysts (Fig. 2). This pattern can be accounted for if we take into account that the random distribution of the -NH2 chain terminal groups in the bulk of the oligomer 1 leads to the formation of H-bonds affecting the interaction between PdC12 and the oligomeric species; in turn this allows for a more facile reduction of Pd2+. This evidence also accounts for the broad reduction profile of PdC12 supported both on carriers 2 and 3 (Fig. 3).

Nevertheless, the decrease of TO upon increase of Pd loading (Table 1) as well as the shift of the TM’s to lower T, indicate that the reducibility of the Pd/ 1 system grows with the Pd loading, as observed for conventional heterogeneous catalysts.

Therefore, the above results clearly indicate that an unusual strong interaction between support and PdC12 precursor greatly stabilises Pd2+ against reduction in Pd catalysts supported on oligomeric aramides.

4.2. TPR pattern of PdCl, supported on different oligomeric matrices

Although the TPR pattern of the Pd/l system (Fig. 2) shows a H2 consumption which is a linear function of the Pd loading (Table 1) , it should be emphasized that such a H2 consumption in any case is more than one order of magnitude larger than that expected on the basis of a theoretical reduction process of the Pd2+ content of the system (Table 1) . The explanation to this finding probably lies in the unusual reactivity of the carrier 1 which consists of chains of aromatic rings bridged by -NH-CO- groups [ 5,6] and acts as a H2 acceptor when submitted to the catalytic


action of reduced Pd, promoting the hydrogenation of the unsaturated matrix. Therefore, a change in the chemical composition of the matrix, realized by

copolymerization of aromatic and alicyclic monomers (see Experimental section), should exhibit a TPR behaviour markedly different from that observed for Pd/l system. The spectra reported in Fig. 3 confirm the role of the matrix in the TPR pattern of Pd catalysts supported on oligomeric aramide. Indeed, a type of behaviour completely opposite to that of the Pd/l system emerges for Pd/2 (Fig. 3b); in the saturated matrix, at T > 150°C a progressively larger contribution of the H2 desorption peak reveals the occurrence of a dehydrogenation process catalytically medi- ated by prereduced Pd’. The intensity of this H2 release peak, comparable with that related to the hydrogenation of support 1 (Table 2), adequately proves the catalytic role of Pd” on the reactivity of the matrix.

According to these findings, Pd/3 represents an intermediate situation between Pd/l and Pd/2. Indeed, this system displays a Pd2+ reduction profile similar to that observed for previous catalysts, even if the related H2 consumption peak shows a less important contribution at high T, marked by the disappearance of the shoulder denoted by TM3 (Fig. 3~). In addition, a negative baseline drift at 210°C indicates a H2 desorption process occurring without any resolved maximum rate ( Td) and then becoming flat-shaped at its maximum values. Taking into account that supports 1,2 and 3 have similar average molecular masses, such an irregular profile can be likely ascribed to the occurrence of two concomitant and opposite processes; namely, the dehydrogenation of saturated C6 rings together with the hydrogenation of aromatic ones, both contained in the matrix 3. This implies that the whole process proceeds at a constant rate in a wide range of T. However, the lower intensity of the desorption process in Pd/3 catalyst with respect to Pd/2 reflects a lower con- centration of saturated C6 rings in the latter system.

Acknowledgements

The authors acknowledge the financial contribution to this work of the Ministers dell' Universitiz e della Ricerca Scientifica and of the Consiglio Nazionale delle Ricerche (Rome, Italy).

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palladium-based catalysts supported on oligomeric aramides. a tpr investigation

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