The Ensemble Effect in Bifunctional Catalysis: Influence of Zinc As Promoter for Pd-H-ZSM-5 Catalysts During the Dehydroalkylation of Toluene with Ethane

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The Ensemble Effect in Bifunctional Catalysis: Influence of Zinc as Promoter for Pd-H-ZSM-5 Catalysts during the Dehydroalkylation of Toluene with Ethane

Daniel Geiß and Yvonne Traa*

DOI: 10.1002/cite.202000129

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. In Memory of Prof. Dr.-Ing. Jens Weitkamp

The ensemble effect, i.e., the fact that the reaction requiring the largest ensemble of surface metal atoms will be most sensitive to promoting with a second metal, has rarely been studied in bifunctional catalysis. In this contribution, it is shown that, during the dehydroalkylation of toluene with ethane on Pd-H-ZSM-5 catalysts with zinc as promoter, the ensemble effect has only an influence on the side reactions on the metal such as hydrogenolysis, but not on the desired acid- catalyzed alkylation reaction. Keywords: Bifunctional catalysis, Dehydroalkylation, Ensemble effect, Zinc as promoter, Zn/Pd-H-ZSM-5 Received: July 06, 2020; revised: September 11, 2020; accepted: January 26, 2021

1 Introduction 2 Experimental Section

The ensemble effect is a well-known phenomenon in het- Zeolite ZSM-5 was synthesized by hydrothermal synthesis. erogeneous catalysis. It was first described in detail by First, a solution of 4.62 or 8.01 g Al(NO3)3 Á 9H2O (98 %, Sachtler and van Santen [1, 2]. The main feature is that, if Riedel-de Hae¨n), 18.51 g tetrapropylammonium bromide parallel catalytic reactions differ in the number of adjacent (>98 % Fluka) and 128.10 g demineralized water was made. surface atoms of the active metal that are required for form- Then, 128.11 g Ludox AS 40 (40 wt % SiO2 in H2O, Sigma ing the respective chemisorption complexes, the reaction Aldrich) was added. To the well dispersed mixture, 158.70 g requiring the largest ensemble of these atoms will be the ammonia solution (25 wt % in water, Riedel-de Hae¨n) was most sensitive to alloying with a second metal [1, 2]. The added. The mixture was homogenized for 15 min at room ensemble effect has mainly been studied for model and/or temperature. The well dispersed mixture was filled into four monofunctional catalyst systems, but rarely in bifunctional stainless-steel autoclaves (volume of 300 cm3). Crystalliza- catalysis [3, 4]. The title reaction, the dehydroalkylation of tion occurred under rotating conditions (0.5 s–1) at 433 K toluene with ethane, is catalyzed by bifunctional catalysts within 5 days. Afterwards, the suspension was filtered and and is believed to occur in two steps, firstly the dehydroge- washed with 200 mL demineralized water, and the solid was nation of ethane on the metal sites and secondly the alkyla- dried at 353 K for 24 h. To remove the template, the catalyst tion of toluene with ethene on the acid sites [5]. For the first was heated slowly (0.66 K min–1) to 393 K under flowing step, the dehydrogenation of alkanes, the ensemble effect nitrogen, this temperature was held for 24 h, then the has been described for Pt-Sn catalysts on non-acidic supp- catalyst was further heated (0.66 K min–1) to 823 K. The gas orts [6]. Thus, the interesting feature of this contribution is was switched to synthetic air, and the temperature was held that the ensemble effect has only an influence on the side at 823 K for another 48 h. reactions on the metal, such as hydrogenolysis, and the first Palladium ion exchange was carried out by adding drop- step of the main reaction but not on the actual alkylation wise under stirring an aqueous solution of Pd(NH3)4Cl2 reaction that occurs on acid sites. Hence, further insight (40.62 wt % Pd, ChemPur) to a suspension of the zeolite in into the ensemble effect in bifunctional catalysis for more complex reactions is possible. – Daniel Geiß, Apl. Prof. Dr. Yvonne Traa [email protected] University of Stuttgart, Institute of Technical Chemistry, Pfaffen- waldring 55, 70569 Stuttgart, Germany.

Chem. Ing. Tech. 2021, 93, No. 6, 1–5 ª 2021 The Authors. Chemie Ingenieur Technik published by Wiley-VCH GmbH www.cit-journal.com Online in color, print in grayscale unless the author(s) contribute to the costs. ’’ These are not the final page numbers! Chemie 2 Communication Ingenieur Technik demineralized water. The mixture was stirred at room tem- approximately 4 through a toluene (> 99.9 %, Merck) satu- perature for 24 h, filtered and dried at 353 K for another rator containing Chromosorb P-NAW (Macherey-Nagel). 24 h. The catalyst was then calcined at 823 K in nitrogen for Nitrogen was used as an internal standard but also to ensu- 24 h and cooled to room temperature. Optionally, the zeo- re that a relatively low n_ ethane=n_ toluene feed ratio of 5 ± 1 lite was suspended in demineralized water, and zinc acetate could be achieved at the high pressure applied. The reaction (C4H6O4 Zn Á 2H2O, 99.0 %, Fluka) was added. The water was carried out at a total pressure of 2.4 MPa and a reaction was carefully removed in a rotary evaporator, thereby im- temperature of (623 ± 2) K. The WHSV (toluene and pregnating the catalyst with Zn. Afterwards, the catalyst ethane) was 1.0 h–1. Product analysis was achieved using an was dried at 353 K for 24 h. For the catalytic experiments, online sampling system, a capillary gas chromatograph and the zeolite powder was pressed without a binder, crushed a CP-PoraPLOT Q column (length: 30 m, inner diameter: and sieved to get a particle size between 0.2 and 0.3 mm. 0.32 mm, film thickness: 20 mm, Chrompack). Two detec- The catalyst was activated in situ, prior to starting the ex- tors in series were employed, namely, a thermal conductivi- periment. To achieve a high dispersion of the noble metal, ty detector followed by a flame ionization detector. Correc- 0.5 g of the catalyst were first heated in flowing synthetic air tion factors for the two detectors were determined (150 cm3min–1) at a rate of 0.25 K min–1 to a final tempera- separately. With ethane as tie substance, the results from ture of 573 K, then it was switched to nitrogen both detectors were combined. The selectivities were calcu- (150 cm3min–1) and heated with a rate of 1.7 K min–1 to a lated from the molar flows. final temperature of 623 K. Afterwards, the catalyst was reduced under a constant stream of hydrogen (150 cm3min–1) at 623 K for 4 h. The catalyst designation 3 Results and Discussion 0.1Zn/0.6Pd-H-ZSM-5 (24) indicates an H-ZSM-5 catalyst with an nSi/nAl ratio of 24, which has been ion-exchanged To start with, Fig. 1 shows the toluene conversions and the with Pd and impregnated with Zn, with Pd and Zn contents selectivities to ethyltoluenes for H-ZSM-5 catalysts with given as wt % related to the dry catalyst. An overview of the nSi/nAl ratios of 24 or 40, containing Pd only. It can be catalysts used in this study and their compositions is given observed, that the selectivities are quite similar for all in Tab. 1. catalysts. By contrast, the toluene conversions are conside- The water content of the zeolites was determined using a rably higher for the 0.8Pd-H-ZSM-5 (40) catalyst with the Setaram Thermogravimetric Analyzer Setsys TG-16/18. An lower aluminum content. It has been observed earlier that optical emission spectrometer (OES) of type Varian with the toluene conversion reaches a maximum at nSi/nAl ratios inductively-coupled plasma (ICP) (Vista-MPX CCD of around 40 [7]. It has been shown that side reactions such ICP-OES) was used for the chemical analysis of the as toluene disproportionation to xylenes and benzene pre- catalysts. The zeolite structures were verified by powder vail at lower nSi/nAl ratios at atmospheric pressure, which X-ray diffraction using a BrukerD8 Advance diffractometer also gives rise to a slightly increased coke formation with CuKa radiation (wavelength of 0.154 nm). The palla- (3.1 % C on 0.9Pd/H-ZSM-5 with an nSi/nAl ratio of 11 and dium dispersion was determined by CO adsorption at 308 K 1.5 % C on 0.9Pd/H-ZSM-5 with an nSi/nAl ratio of 41) and using Quantachrome Autosorb equipment. increased deactivation of the catalyst [7]. The conversions Catalytic experiments were performed in a flow-type are typically higher and more stable and the selectivities to apparatus with a fixed-bed reactor from stainless steel. Eth- ethyltoluenes lower at higher pressures [8]. Therefore, at ane (99.95 vol %, Westfalen AG) and nitrogen (99.999 vol %, the higher pressure of 2.4 MPa applied in this study, the Westfalen AG) were fed with an n_ nitrogen=n_ ethane ratio of conversion change is more pronounced with increasing nSi/ nAl ratio, whereas the selectivities to ethyltolue- Table 1. Composition of the catalysts used as determined with ICP-OES and nes do not change considerably in the limited thermogravimetry. range of nSi/nAl ratios because they are lower at higher pressure [8] and, due to the strongly inc- Catalyst nSi/nAl mPd/mdry catalyst [%] mZn/mdry catalyst [%] reased conversion, different effects on the selec- 0.9Pd-H-ZSM-5 (22) 22 0.85 – tivities interfere with each other. 0.1Zn/0.9Pd-H-ZSM-5 (23) 23 0.88 0.13 Fig. 2 shows that by promoting palladium with zinc not only the selectivity in the desired alkyla- 0.9Pd-H-ZSM-5 (24) 24 0.94 – tion reaction is increased, but also the conver- 0.1Zn/0.8Pd-H-ZSM-5 (24) 24 0.75 0.14 sion. Thus, the yield of the desired ethyltoluenes 0.1Zn/0.6Pd-H-ZSM-5 (24) 24 0.63 0.14 is more than doubled from values around 5 % for the monometallic Pd catalyst to values 0.8Pd-H-ZSM-5 (40) 40 0.79 – around 13 % for the promoted catalyst [9]. This 0.1Zn/0.4Pd-H-ZSM-5 (40) 40 0.37 0.14 holds not only for different experiments with catalysts from the same batch but also for a freshly 0.9Zn/0.4Pd-H-ZSM-5 (40) 40 0.38 0.89 prepared catalyst with slightly different compo-

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25 25 a) a)

20 20 % % / / n n o o i 15 i 15 s s r r e e v v n n o o c c e 10 e 10 n n e e u u l l o o T T 0.9Pd-H-ZSM-5 (24) run 1 5 5 0.9Pd-H-ZSM-5 (24) run 2 0.1Zn/0.6Pd-H-ZSM-5 (24) run 1 0.9Pd-H-ZSM-5 (24) run 1 0.8Pd-H-ZSM-5 (40) run 1 0.1Zn/0.6Pd-H-ZSM-5 (24) run 2 0.9Pd-H-ZSM-5 (24) run 2 0.8Pd-H-ZSM-5 (40) run 2 0.1Zn/0.8Pd-H-ZSM-5 (24) 0 0 0246810 0246810 Time on stream / h Time on stream / h

b) b) 80 80 % % / / s s e e

n 60

n 60 e e l u l u o o t t l l y y h h t t e 40 e o 40 o t 0.9Pd-H-ZSM-5 (24) run 1 t y y t t i 0.9Pd-H-ZSM-5 (24) run 2 i v v i i t t

c 0.8Pd-H-ZSM-5 (40) run 1 c e e l 0.8Pd-H-ZSM-5 (40) run 2 l e 20 e 20 S S 0.1Zn/0.6Pd-H-ZSM-5 (24) run 1 0.9Pd-H-ZSM-5 (24) run 1 0.1Zn/0.6Pd-H-ZSM-5 (24) run 2 0.9Pd-H-ZSM-5 (24) run 2 0.1Zn/0.8Pd-H-ZSM-5 (24) 0 0 0246810 0246810 Time on stream / h Time on stream / h

Figure 1. a) Toluene conversions and b) selectivities to ethyl- Figure 2. Effect of zinc as a promoter for Pd-H-ZSM-5 (24) cata- toluenes on Pd-H-ZSM-5 catalysts with nSi/nAl ratios of 24 and 40 lysts on a) toluene conversions and b) selectivities to ethyl- (run 1 and run 2 are reproduction experiments conducted with toluenes (run 1 and run 2 are reproduction experiments fresh catalyst). conducted with fresh catalyst). sition. The fact that bimetallic systems have an enhanced sites is favored. This explanation is supported by the strong- activity has also been described earlier and explained by a ly reduced selectivity to methane (not shown) – a typical change in selectivity [1, 2]. product of hydrogenolysis – on the Zn-containing catalyst, For dehydrogenation, a monoadsorbed complex is suffici- and this has been observed before on Pt-Sn/H-SAPO-11 ent, which can be easily desorbed. However, the monoad- [4]. In addition, we prepared two more samples for further sorbed complex can also become multiply adsorbed, so that characterization: The Pd dispersion increased from 25 % for nondestructive desorption might be difficult, and hydroge- 0.9Pd-H-ZSM-5 (22) to 33 % for 0.1Zn/0.9Pd-H-ZSM-5 nolysis might become the preferred way to regenerate the (23), thus showing that promoting the catalyst with Zn free sites of the catalyst [1, 2]. Hence, larger ensembles are decreases the size of the Pd ensembles. However, in addition required for C–C bond-breaking reactions compared to to the effects described above, Zn or ZnO (formed from C–H bond cleavage as in dehydrogenation. Thus, promo- zinc acetate during calcination) could be, at least partially, ting Pd with Zn will lower the ratio of poly- to monoadsor- ion exchanged into the H-ZSM-5 zeolite during the activa- bed molecules and have an effect on catalytic selectivity tion of the catalyst [11], and Zn-H-ZSM-5 is also active in [1, 2]. For the Zn-Pd system examined in this study, it has ethane dehydrogenation and aromatization [11]. been proven on model Zn/Pd(111) catalysts that even small Fig. 3 shows the influence of increasing the nZn/nPd ratio amounts of Zn destabilize CO bonding on threefold Pd sites and decreasing the Pd content for the zeolite with the high- to the point that adsorption on atop sites becomes possible er nSi/nAl ratio of 40. For an nZn/nPd ratio of 0.6 [10]. By reducing the activity of the catalyst for metal- (0.1Zn/0.4Pd-H-ZSM-5 (40)), the selectivity to ethyl- catalyzed side reactions such as hydrogenolysis, the alkyla- toluenes is still increased, whereas the toluene conversion tion of toluene with the in situ produced ethene on the acid is slightly decreased. For an nZn/nPd ratio of 3.7

25 4 Conclusions a) In summary, it has been shown that promoting Pd-H-ZSM- 20 5 catalysts with Zn for the dehydroalkylation of toluene %

/ with ethane can considerably increase the yield of the n o i 15 desired alkyl aromatics produced on the acid sites of the s r e

v bifunctional catalyst and repress side reactions on the metal, n o

c such as hydrogenolysis. This positive effect is only observed

e 10 n

e if Zn is used in relatively small amounts. Thus, this new u l o

T example for the positive influence of the ensemble effect in 0.8Pd-H-ZSM-5 (40) run 1 5 bifunctional catalysis shows that significant amounts of 0.8Pd-H-ZSM-5 (40) run 2 0.1Zn/0.4Pd-H-ZSM-5 (40) noble metals can be saved by using promoters. 0.9Zn/0.4Pd-H-ZSM-5 (40) 0 0246810 The research leading to these results received funding Time on stream / h from the EC 7th Framework Program through the Collaborative Project NEXT-GTL under agreement n° 229183. The authors thank Dr. Sarah Sealy for 80 b) valuable contributions to the preliminary work, Barbara

% Gehring for preparing some catalyst samples and Dennis / s

e Wan Hussin for technical help. Open access funding

n 60 e u l enabled and organized by Projekt DEAL. o t l y h t e 40 o t 0.8Pd-H-ZSM-5 (40) run 1 y t i 0.8Pd-H-ZSM-5 (40) run 2 Symbols used v i t

c 0.1Zn/0.4Pd-H-ZSM-5 (40) e l 0.9Zn/0.4Pd-H-ZSM-5 (40) e 20

S n [mol] molar amount n_ [mol s–1] molar flow rate m [g] mass 0 –1 0246810 WHSV [h ] weight hourly space velocity Time on stream / h References Figure 3. Effect of zinc as a promoter and Pd contents on a) toluene conversions and b) selectivities to ethyltoluenes on Pd-H-ZSM-5 (40) catalysts (run 1 and run 2 are reproduction [1] W. M. H. Sachtler, Le Vide 1973, 164, 67–71. experiments conducted with fresh catalyst). [2] W. M. H. Sachtler, R. A. van Santen, Adv. Catal. 1977, 26, 69–119. [3] D. L. Hoang, S. A.-F. Farrage, J. Radnik, M.-M. Pohl, M. Schneider, H. Lieske, A. Martin, Appl. Catal., A 2007, 333, 67–77. (0.9Zn/0.4Pd-H-ZSM-5 (40)), the effect is more significant: [4] T. Komatsu, H. Ikenaga, J. Catal. 2006, 241, 426–434. The selectivity to ethyltoluenes is reduced and converges to [5] G. Caeiro, R. H. Carvalho, X. Wang, M. A. N. D. A. Lemos, F. Le- the value of the monometallic Pd catalyst. The toluene con- mos, M. Guisnet, F. Ramoˆa Ribeiro, J. Mol. Catal. A: Chem. 2006, version is halved. This demonstrates that promoting Pd 255, 131–158. with Zn has a very positive effect as long as Zn is used in [6] K. J. Caspary, H. Gehrke, M. Heinritz-Adrian, M. Schwefer, in relatively small amounts. If there is more Zn than Pd in the Handbook of Heterogeneous Catalysis (Eds: G. Ertl, H. Kno¨zinger, F. Schu¨th, J. Weitkamp), 2nd ed., Vol. 7, Wiley-VCH, Weinheim catalyst, Zn has a negative effect. These results are in line 2008, 3206–3229. with similar results reported for dehydrogenation of cyclo- [7] A. Bressel, T. Donauer, S. Sealy, Y. Traa, Microporous Mesoporous hexane on bimetallic Ni catalysts where highest selectivities Mater. 2008, 109, 278–286. and conversions were observed for nMe/nNi ratios of 0.1 [8] D. Singer, S. A. S. Rezai, S. Sealy, Y. Traa, Ind. Eng. Chem. Res. [12]. If the second metal is present in higher amounts, acti- 2007, 46, 395–399. vity and selectivity to the dehydrogenation product are [9] Y. Traa, D. Geiss, Patent EP2726445B9, 2019. decreased to values below the one on the monometallic Ni [10] E. Jeroro, M. P. Hyman, J. M. Vohs, Phys. Chem. Chem. Phys. 2009, 11, 10457–10465. catalyst. Coking and hydrogenolysis reactions are favored. [11] J. Heemsoth, E. Tegeler, F. Roessner, A. Hagen, Microporous The effect can be explained by blocking of the active metal Mesoporous Mater. 2001, 46, 185–190. due to the higher amounts of the second metal [12]. This [12] H. D. Lanh, N. Khoai, H. S. Thoang, J. Vo¨lter, J. Catal. 1991, 129, has also been described for Pt-Bi systems [13]. For the ca- 58–66. [13] C. T. Campbell, J. M. Campbell, P. J. Daton, F. C. Henn, J. A. Ro- talyst used here, the system approaches for higher nZn/nPd ratios Zn-H-ZSM-5, which is known to be a good catalyst driguez, S. G. Seimanides, J. Phys. Chem. 1989, 93, 806–814. [14] V. B. Kazansky, I. R. Subbotina, N. Rane, R. A. van Santen, E. J. for the aromatization of light paraffins [11, 14]. M. Hensen, Phys. Chem. Chem. Phys. 2005, 7, 3088–3092.

DOI: 10.1002/cite.202000129

14 The Ensemble Effect in Bifunctional Catalysis: Influence of Zinc 12

as Promoter for Pd-H-ZSM-5 Catalysts during the Dehydroalkylation % /

s 10 0.1Zn/0.6Pd-H-ZSM-5 (24) run 1 e of Toluene with Ethane n e 0.1Zn/0.6Pd-H-ZSM-5 (24) run 2 u l 8 0.1Zn/0.8Pd-H-ZSM-5 (24) o t D. Geiß, Y. Traa* l y h t

e 6 f o d l e i 4 Communication: This example of the ensemble effect during the dehydroalkylation of Y 0.9Pd-H-ZSM-5 (24) run 1 toluene with ethane shows that hydrogenolysis as side reaction on the metal is suppressed, 2 0.9Pd-H-ZSM-5 (24) run 2 which favors the alkylation reaction on acid sites. Thus, the yield of ethyltoluenes is more 0 024681012 than doubled from values » 5 % for the Pd catalyst to values » 13 % for the Zn-promoted Time on stream / h catalyst...... ¢

Chem. Ing. Tech. 2021, 93, No. 6, 1–5 ª 2021 The Authors. Chemie Ingenieur Technik published by Wiley-VCH GmbH www.cit-journal.com