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Long-time Stability vs. Deactivation of Pd-Ag/Al2O3 EggShell Catalysts in Selective Hydrogenation of Acetylene Martin Kuhn, Martin Lucas, and Peter Claus Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2015 Downloaded from http://pubs.acs.org on June 13, 2015

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Long-time Stability vs. Deactivation of Pd-Ag/Al2O3 Egg-Shell Catalysts in Selective Hydrogenation of Acetylene Martin Kuhn, Martin Lucas, Peter Claus*

Technische Universität Darmstadt, Department of Chemistry, Ernst-Berl-Institute, Chemical Technology II, Alarich-Weiss-Straße 8, D-64287 Darmstadt, Germany. *Corresponding author: [email protected], +49 6151 16-5369

Abstract Industrial relevant Pd-Ag/Al2O3 egg-shell catalysts with varying Pd/Ag mole ratios were prepared, characterized (CO chemisorption, DRIFTS, EPMA, STEM, coupled with EELS and EDX, as well as TPR) and tested for selective hydrogenation of acetylene under tail-end conditions. Characterization suggests a reduction of the total number of Pd surface atoms nPd,surf as well as an isolation of large Pd ensembles. The effect of silver addition on by-product formation and deactivation behavior was investigated, representing a higher long time stability of Pd5-Ag95/Al2O3 in comparison to silver free Pd100/Al2O3 eggshell catalysts, due to a decrease of green oil and coke formation. The selectivity to C6 compounds is a fundamental indicator for long-term stability of palladium catalysts which is often overlooked in the literature. Furthermore this study implicates selective hydrogenation of acetylene as structure sensitive reaction originating from a geometric effect. Keywords Palladium, Silver, egg-shell catalysts, selective acetylene hydrogenation, deactivation

1. Introduction Ethylene is one of the most important petrochemical bulk chemicals with a production rate of 1

approximately 120 Mt/y. Over 50 % are used for polymerization to polyethylene with different properties. Ethylene is mainly produced by steam cracking of naphtha at high temperature above 800 °C, impurities e.g. acetylene are formed. The acetylene content has to be reduced to concentrations lower than 1 ppmv to avoid irreversible damage of catalysts used in downstream processes of the steam cracker. 2

Furthermore, the presence of acetylene also affects the quality of the polyethylene produced. The catalytic removal is carried out by selective hydrogenation of acetylene to ethylene. Pd-Ag/Al2O3 egg-shell catalysts with low metal loadings are commercially used at front-end and tail-end conditions. The target improvement of these catalysts is to increase the selectivity to ethylene, raising the amount of ethylene gain and to force up the long term stability. Firstly, Bond et al.

4

3

reported about the use of Pd catalysts in selective hydrogenation of acetylene. A

selective Pd catalyst was associated with small Pd ensembles which favor acetylene adsorption but 1


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5

restrict ethylene adsorption. Furthermore, the selectivity to ethylene is enhanced by an interplay between Pd carbides

6

and hydrides

7, 8

. Borodzinski reported about an increasing turnover-frequency (TOF) with

Pd particle size up to 26 nm over Pd/SiO2 catalysts and the Pd particle size was found to have no 9

influence on the adsorption strengths of acetylene and ethylene as well as the reaction mechanism. The structure sensitivity of only Pd containing catalysts was discussed in terms of geometric and electronic 9, 10

effects.

Promoting Pd based catalysts was reported to increase the ethylene selectivity, reduce the 5

yield of green oil and improve the properties of traditional catalysts. Especially the use of group I B metals such as Cu

11

, Ag

8, 12-14

and Au

15, 16

should be mentioned. Pd-Ag/Al2O3 egg-shell catalysts are used in

3

industry since 1980s until today. Egg-shell catalysts are characterized by a distribution of the catalytic active metals in the outer sphere of the supporting material. To control the metal distribution during impregnation several factors of influence are important like the used precursor, the solvent and drying conditions.

17, 18

Using egg-shell catalysts in the selective hydrogenation of acetylene, the selectivity to the

target product ethylene can be enhanced, avoiding the consecutive hydrogenation of ethylene to ethane, as well as the pressure drop and the influence of heat transport resistance can be reduced in industrial application. Q. Zhang et al.

8

reported about a synergetic effect of Pd and Ag dispersed on Al2O3 due to

alloying of Pd and Ag. An ensemble effect as well as a ligand effect was found using XRD and XPS. Moreover Y. Zhang et al.

19

studied the influence of Ag and Au deposited on a 1.85 wt.% Pd/SiO2. They

found the selectivity to ethylene and the TOF to acetylene to be enhanced by high coverages of Ag and Au on Pd, explained by a transition from strongly adsorbed acetylene to a weakly bonded π-species, due to a formation of small Pd ensembles. Recently, investigations of new catalyst systems for selective hydrogenation of acetylene reported about intermetallic Pd-Ga 23, 24

MgO/Al2O3.

20-22

compounds and non-precious metal catalysts as Ni-Zn impregnated on

Furthermore, the application of SCILL (solid catalysts with ionic liquid layer) type catalysts

is reported, which enhance the ethylene selectivity by reducing the consecutive hydrogenation of ethylene to ethane.

25

Scheme 1 shows the complexity of the selective hydrogenation of acetylene. The hydrogenation of acetylene to ethylene is the desired reaction, but simultaneously ethylene is hydrogenated to ethane, which leads to a decrease in economy of the overall process. In addition, hydrooligomerization of acetylene gives rise to the formation of butenes and butadiene. Butadiene is proposed to be the precursor of heavy hydrocarbons

26

, so called green oil. Furthermore the green oil causes the formation of solid

coke, which is deposited on the active sites of the catalytic surface and leading to a decrease in activity.

3

Scheme 1. Reaction network of the selective hydrogenation of acetylene. 2



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Isolation of active Pd surface atoms by reducing Pd-Pd coordination number and increasing Pd-Pd distance affect the adsorption configurations of the unsaturated hydrocarbons. For acetylene two important configurations are reported, the weak π-bonding on top on a Pd atom and the di-σ-bonding on two neighboring Pd atoms. Only weakly π-bonded acetylene is selectively hydrogenated to ethylene whereupon the di-σ-bonded species of acetylene lead to the formation of ethane.

8, 27

Additionally, another

reaction path for ethane formation over hydrogen spill-over is discussed. The activated hydrogen is transferred to the support over the coke depositions and reacting with ethylene to ethane.

11, 28-31

Fig. 1 shows a cross section of a Pd-Ag/Al2O3 egg-shell catalyst before and after 100 h time on stream (TOS). Understanding and controlling of the deactivation behavior is a major field of research in industrial application to increase the lifetime of the used catalysts and to extend the TOS between each regeneration cycle. Furthermore, the fransfer of findings from model systems to the industrial use is an important aspect.

Figure 1. A cross section of a Pd-Ag/Al2O3 egg-shell catalyst fresh (left) and used (right) after 100 h TOS. Although the selective hydrogenation on Pd-Ag/Al2O3 egg-shell catalysts is a mature industrial reaction, active ensembles and the role of Ag, as well as the influence on deactivation behavior are still object of research especially for catalysts with industrial relevance.

2. Experimental 2.1 Catalyst preparation Pd-Ag/Al2O3 egg-shell catalysts with 0.035 wt% Pd and different Ag loadings were prepared by 2

impregnation of Al2O3 cylindrical pellets (4.5 mm x 4.5 mm, 13 m /gcat, 0.49 ml/gcat) with aqueous palladium nitrate and/or silver nitrate solutions simultaneously followed by calcination at 500 °C and liquid phase reduction with hydrazine at room temperature. The average breadth of the catalytic active zone, the so called shell, is about 350 µm. The nominal metal loadings and the atomic ratios of Pd/Ag were confirmed by ICP-OES and XRF spectroscopy and are summarized in Table 1.

3


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Table 1. Metal loadings and Pd/Ag mole ratios of the prepared Pd-Ag/Al2O3 egg-shell catalysts. a

b

Catalyst

Pd [wt.-%]

Ag [wt.-%]

Pd/Ag mole ratio

Pd [at.-%]

Pd100/Al2O3

0.035

-

100/0

100

Pd95-Ag5/Al2O3

0.035

0.002

95/5

95

Pd80-Ag20/Al2O3

0.035

0.009

80/20

80

Pd70-Ag30/Al2O3

0.035

0.015

70/30

70

Pd50-Ag50/Al2O3

0.035

0.035

50/50

50

Pd30-Ag70/Al2O3

0.035

0.083

30/70

30

Pd20-Ag80/Al2O3

0.035

0.142

20/80

20

Pd5-Ag95/Al2O3

0.035

0.674

5/95

5

a

b

Confirmed by ICP-OES analysis. Confirmed by XRF spectroscopy.

2.2 Catalyst characterization Chemical analyses were obtained by inductive coupled plasma optical emission spectroscopy (ICP-OES, Varian 715-ES ICP-emission spectrometer) for Pd and by X-ray fluorescence spectroscopy (XRF, Bruker AXS S4 Explorer) for Ag. Electron probe micro analyses (EPMA, SEMQ ARL) was performed to determine the metal distribution 3

along the pellet diameter in accordance to . Scanning transmission electron microscopy (STEM) was undertaken in a JOEL JEM ARM 200F microscope operated at 200 kV. Energy dispersive X-ray spectroscopy (EDX) was conducted with a JED 2300T detector and a Gatan Enfina detector was used to perform electron energy loss spectroscopy (EELS). The CO uptake was determined by CO pulse chemisorption on TPD/R/O 1100 (Porotec) using thermal conductivity detector (TCD). The catalyst samples were pretreated in 50 ml/min hydrogen flow (H2 N50; Air Liquide) and 200 °C for 1 h. Then the samples were cooled down to 0 °C in hydrogen flow. 10 pulses of 0.473 ml CO (CO N37, Air Liquide) were introduced in 30 ml/min flow of hydrogen with 12 min between the pulses. The decrease of the TCD signal area indicates the CO uptake corresponds with the CO content nCO chemisorbed on the samples. Temperature programmed reduction of the exclusive calcined and crushed catalysts were carried out using a TPD/R/O 1100 (Porotec) with a TCD. Spectra were collected in a temperature range of -10 °C to 600 °C using a heating rate of 3 K/min. TPR results are presented per gram of sample. DRIFTS of adsorbed CO on uncrushed egg-shell catalysts were performed using an Equinox55 spectrometer (Bruker). The samples were first heated in 10 ml/min nitrogen flow (N2 N50, Air Liquide) at 150 °C and in-situ reduced in a flow of 10 ml/min hydrogen (H2 N50; Air Liquide) at 120 °C. The spectra -1

(64 scans, 4 cm resolution) were collected after 1 h exposure of 1.3 vol.-% CO in nitrogen (CO N47 in N2 N50, Air Liquide) using KBr as reference. Results are presented as difference spectra relative to the initial scan collected prior to exposure of CO. For integration and determination of the ratio between multi (all adsorption bands not representing 1:1 adsorption stoichiometry) to linear bonded CO Am/Al the peak 4



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Page 16 of 32

shape was fitted and integrated using Gauss-Lorenz equation. Additionally the integrals are corrected using extinction coefficients.

32

The increase of mass of the egg-shell catalysts after reaction was determined by weighing the catalyst pellets before and after each catalyst test.

2.3 Catalyst testing The selective hydrogenation of acetylene in an ethylene-rich feedstock under tail-end conditions was performed in the Advanced TEMKIN-reactor. Our development of the latter allows very precise measurements of conversion and selectivity of cylindrical Pd-Ag/Al2O3 egg-shell catalysts, not influenced by external mass and heat transport limitations. Details of experimental setup and validation are already published.

33, 34

About 3.5 g of egg-shell catalysts were reduced in-situ in 100 Nml/min hydrogen flow (H2

N50; Air Liquide) for 1 h, followed by a run-in period at standard conditions of 30 °C, 10 bar excess -1

pressure and a gaseous hourly space velocity (GHSV) of 4000 h for 10 h. The test gas consists of 1 vol.% acetylene (C2H2 N26; Air Liquide), 1 vol.% hydrogen (H2 N30; Air Liquide), 1 vol.% propane (C3H8 N25; Air Liquide), as internal standard, 30 vol.% ethylene (C2H4 N30; Air Liquide) and 67 vol.% argon (Ar N50; Air Liquide). The selectivity-conversion behavior of the catalysts was investigated by varying the modified residence time τmod, constituting a ratio of catalyst mass to molar flux of acetylene. Additionally, the temperature was raised stepwise up to 60 °C causing formation and fouling of hydrocarbons and coke. Pd100/Al2O3 and Pd5-Ag95/Al2O3 were investigated under standard conditions over 100 h for long-term tests. The reaction temperature has to be adjusted observing a conversion of about 70 % at the beginning of the test. The composition of the gas stream at the outlet of the reactor was analyzed online by GC (HP 6890 Plus) equipped with a capillary column (HP-Plot, Al2O3-S-deactivated) and a flame ionization detector (FID) every 15 min. The hydrocarbons detected by FID were integrated by the software “ChemStation” (Agilent) 35

and normalized using substance specific correction factors.

Conversion of acetylene was calculated as

the amount after reaction divided by the amount introduced (1). = 1 −

, ∙ 100%

,

(1)

Selectivity to the byproducts ethane, C4 and C6 were determined as the amount of formed byproducts divided by the amount of acetylene converted (2-4). The selectivity to ethylene was determined indirectly, using the selectivity of the by-products ethane and the C4 and C6 components (5), because changes of the ethylene concentration cannot be detected exactly. =

=

3

, ∙ 100%

,

2 ∙ ∑

,

,

∙ 100%


(3) 5

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# =

3 ∙ ∑ #, ∙ 100%

,

= 100 − − − #

(4)

(5)

3. Results and discussion Using industrial relevant Pd-Ag/Al2O3 egg-shell catalysts in selective hydrogenation of acetylene the metal distribution plays an important role. Under reaction engineering aspects, it must be kept in mind that activity and selectivity can be significantly influenced by pore diffusion, depending on the thickness of the active shell. Therefore EPMA was performed to determine the metal distribution. The Pd- and Ag-EPMA in dependence of the pellet diameter is shown in Fig. 2 for Pd100/Al2O3 and Pd80-Ag20/Al2O3. The thickness of the active shell is defined as the depth of Pd. It is about 350 µm for all catalysts and in the range of 3

industrial application. A homogeneous distribution for Pd and Ag is obtained.

Figure 2. Pd- and Ag- concentration in dependence of the pellet diameter for the catalysts Pd100/Al2O3 (left) and Pd80-Ag20/Al2O3 (right), measured by EPMA. XRD is not suitable for investigation of the bimetallic character of the Pd-Ag particles formed on the support, due to the low metal loadings. Therefore STEM coupled with EELS and EDX was used. The data are exemplified for Pd70-Ag30/Al2O3 in Fig. 3. A qualitative evidence for Ag could be drawn by EELS (Fig. 3 36

b) on the basis of the edge at 367 eV (Ag M4,5).

The EDX spectrum (Fig. 3 c) reveals the existence of

bimetallic Pd-Ag particles. No separate Pd and Ag could be found for Ag containing catalysts. The average crystallite size rises slightly with increasing Ag content from 4.22 nm ± 1.57 nm for at Pd100/Al2O3 to 7.87 nm ± 3.20 nm for Pd5-Ag95/Al2O3. The deviations are within the error. For more information see supplementary information.

6



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Page 18 of 32

Figure 3. Example of investigation by STEM a), EELS b) and EDX c) for Pd70-Ag30/Al2O3.to prove bimetallic Pd-Ag particles.

TPR using hydrogen was performed on Pd70-Ag30/Al2O3 (a), Pd100/Al2O3 (b) and Ag100/Al2O3 (d) calcined at 500 °C and Pd100/Al2O3 (c) calcined at 400 °C (Fig. 4). The TPR profile for Ag100/Al2O3 (d) shows a broad reduction peak at approximately 92 °C, reduction of AgO and Ag2O is reported in a range of 70 °C to 130 8

°C in literature. The spectra for Pd100/Al2O3 (b) shows a reduction peak at 39 °C for PdO and a negative feature at 62 °C indicating decomposition of palladium hydride which readily forms up exposure to hydrogen at ambient temperature.

37

Furthermore, two broad reduction features are observed at 177 °C

and 284 °C associated with reduction of palladium species which are harder to reduce due to strong 37, 38

interaction with the supporting material Al2O3.

Calcination at 400 °C instead of 500 °C causes a shift of

the PdO reduction feature to 16 °C. Additionally, the palladium hydride peak been much more 7

pronounced. These observations are influenced by changes in particle size and/or morphology. Instead of the monometallic Pd100/Al2O3 (b) bimetallic Pd70-Ag30/Al2O3 sample (a) shows a shift of the main reduction peak to higher temperature of 45 °C and no evidence of palladium hydride decomposition. Results indicate interaction of Pd and Ag in the bimetallic Pd-Ag/Al2O3 egg-shell catalysts and, thus, reduced palladium hydride formation, which is responsible for formation of ethane in selective hydrogenation of acetylene.

3, 8

7


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Figure 4. TPR profiles of (b) Pd100/Al2O3, (d) Ag100/Al2O3 and (a) Pd70-Ag30/Al2O3 calcined at 500 °C and (c) Pd100/Al2O3 calcined at 400 °C. The CO uptake nCO was measured by CO pulse chemisorption presenting an decrease from 1.92 µmolCO/gcat at Pd100/Al2O3 to 0.16 µmolCO/gcat at Pd5-Ag95/Al2O3 (Table 2). CO is chemisorbed on palladium only and not on silver. These results indicate a decrease of Pd surface atoms by successive addition of Ag. For catalyst Pd100/Al2O3 with a dispersion DPd of 46.2 % and a mean Pd particle size of 2.4 nm can be calculated out of CO chemisorption data. For calculation the amount of adsorbed CO was corrected using DRIFTS experiments described above. The dispersion DPd and Pd particle size cannot be determined for the Ag containing catalysts because silver covers the palladium surface particularly. DRIFTS experiments with CO were carried out with the objective of identifying the presence of linear and multi adsorbed species on the catalytic surface. Fig. 5 shows the DRIFT spectra of prepared egg-shell catalyst after exposure of CO at 45 °C for 1 h. The spectra of Pd100/Al2O3 (a) shows the presence of two -1

main forms adsorbed CO on Pd. The band at 2079 cm is ascribed to CO linearly adsorbed (on-top) on 0

Pd , corresponding to a CO/Pd ratio of 1.

39-41

The broad band between 2000 cm

-1

and 1820 cm

-1

is

assigned to multi bonded CO on Pd, presenting an overlapping of the bands described below. The region -1

-1

between 2000 cm and 1975 cm can be ascribed to bridge CO on open (100) type facets (CO:Pd=1:2) 8



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

or compressed bridge CO on Pd (CO:Pd=2:3)

40-42

-1

40-43

-1

The band between 1900 cm and 1800 cm 43, 44

is usually ascribed to CO adsorbed at 3-fold hollow sites (CO:Pd=1:3). Pd CO

45, 46

-1

and the region between 1950 cm and 1910 cm can

be attributed to isolated bridged CO on Pd (CO:Pd=1:2). +

Page 20 of 32

No band at 2127 cm

-1

-1

for

at all catalysts and no interaction with the only Ag catalysts (not shown) can be observed.

45

The spectra of the bimetallic Pd-Ag/Al2O3 egg-shell catalysts with varying Pd/Ag ratios are shown in Fig. 5 (b) to (h), here the two main bands of linear and multi bonded CO on Pd can be observed again. However, it is found that the IR band for the terminal CO shifts with increasing Ag content from 2079 cm

-1

-1

of Pd100/Al2O3 (a) to 2040 cm of Pd5-Ag95/Al2O3 (h). This hypsochromic shift is induced by a stronger Pd-1

CO bond result in a blue shift of CO stretching against the free CO (2143 cm ). The reason for this observation is discussed controversial in literature, constitute by dipole-dipole interactions electronically effect of Ag addition

45, 50

47-49

and an

. The dipole-dipole interactions are mainly accepted and implicate

an increase of the distance of the CO molecules on the Pd surface, due to isolation of lager Pd ensembles by increasing Ag addition, in the experiments presented here. A bathochromic shift is observed for the broad multi band by increasing Ag content and can be constitute by a transition from mainly CO adsorbed on 3-fold hollow sites (CO:Pd=1:3) to bridged CO (CO:Pd=1:2). The ratios Am/Al between the multi bond (all adsorption bands not representing 1:1 adsorption stoichiometry) and terminal CO on Pd are reported in Table 2. The results demonstrate an isolation of large Pd ensembles by successive addition of Ag to Pd-Ag/Al2O3 egg-shell catalysts at constant Pd loading (ensemble effect). The fluctuating absorbance in the spectra shown in Fig. 2 is caused by using industrial relevant egg-shell catalysts with a rough surface and low metal loadings of 0.035 wt.% Pd which is distributed in an active shell of about 350 µm. The Pd concentration on the surface of the egg-shell catalysts is about 0.1 wt.%, measured with EPMA (Fig. 2).

Table 2. CO chemisorption, CO DRIFTS results and the determined amount of Pd surface atoms nPd,surf of the prepared Pd-Ag/Al2O3 egg-shell catalysts with different Pd/Ag atomic ratios. Catalyst

nCO -1 a

[µmolCO gcat ]

Am/Al

b

nPd,surf -1 c

[µmolPd gcat ]

Pd100/Al2O3

1.92

0.24

2.30

Pd95-Ag5/Al2O3

1.92

0.22

2.26

Pd80-Ag20/Al2O3

1.69

0.19

1.96

Pd70-Ag30/Al2O3

1.66

0.21

1.94

Pd50-Ag50/Al2O3

1.38

0.16

1.57

Pd30-Ag70/Al2O3

0.90

0.16

1.03

Pd20-Ag80/Al2O3

0.49

0.15

0.55

Pd5-Ag95/Al2O3

0.16

0.10

0.17

a

Determined by CO chemisorption.

b

Ratio of multi to linear bonded CO calculated by CO DRIFTS using extinction coefficients

32 c

.

Calculated using equation (6).

9


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Figure 5. DRIFT spectra of (a) Pd100/Al2O3, (b) Pd95-Ag5/Al2O3, (c) Pd80-Ag20/Al2O3, (d) Pd70-Ag30/Al2O3, (e) Pd50-Ag50/Al2O3, (f) Pd30-Ag70/Al2O3, (g) Pd20-Ag80/Al2O3 and (h) Pd5-Ag95/Al2O3 after 1 h CO exposure at 45 °C.

Using the results of CO chemisorption and CO DRIFTS experiments an effective number of Pd surface atoms nPd,surf can be calculated, assuming multi-bonded CO is bonded 2-fold:

%,&'

* +1 * = n) ∙ * +1 * 2∙

(6)

The results are summarized in Table 2 and indicating an almost linearly reduction of the number of nPd,surf with increasing amount of Ag added. The characterization suggests a blocking of large Pd ensembles by Ag, being responsible for hydrogenation of ethylene to ethane and for the dimerization of acetylene to C4 compounds

51

, which are disadvantageous for ethylene selectivity and long-term stability.

The catalyst testing was conducted using the Pd-Ag/Al2O3 egg-shell catalysts at industrial tail-end conditions. The conversions of the Pd-Ag/Al2O3 egg-shell catalysts with different Pd/Ag ratios are shown in Fig. 6. The acetylene conversion decreases with increasing Ag content and decreasing Pd/Ag ratio as expected. Pd100/Al2O3 (a) shows the highest conversions of about 82 %, decreasing to 35 % at Pd5-1

Ag95/Al2O3 (h) at a modified residence time of about 681 gcat h molacetylene , respectively. These results 10



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agree with the catalyst characterization described above. Addition of Ag to Pd /Al2O3 egg-shell catalysts with constant Pd loading decreases the overall activity of the catalysts due to reduction of the total number of Pd surface atoms nPd,surf.

Figure 6. Conversion plotted vs. the modified residence time τmod (left) and Sethylene-Xacetylene diagram (right) of the Pd-Ag/Al2O3 egg-shell catalysts with varying Pd/Ag ratios (a) Pd100/Al2O3, (b) Pd95-Ag5/Al2O3, (c) Pd80-Ag20/Al2O3, (d) Pd70-Ag30/Al2O3, (e) Pd50-Ag50/Al2O3, (f) Pd30-Ag70/Al2O3, (g) Pd20-Ag80/Al2O3 and (h) Pd5-Ag95/Al2O3 (T = 30 °C, p = 10 bar). In the literature the selectivity to ethylene is determined on the basic of the selectivity of ethane and the C4 3

compounds , but no correlations can be drawn to deactivation behavior. Therefore the detection of further byproducts like Benzene, C6H10 and C6H12 is necessary and summarized in selectivity to C6 compounds. Here, the selectivity to ethylene is calculated using the selectivity to ethane, C4 and C6 compounds, illustrated in Fig. 6. The lowest selectivity of 64 % is observed for the pure Pd100/Al2O3 egg-shell catalyst (a) at about 50 % conversion. Addition of Ag affects an increase of the observed selectivity to ethylene, e.g. the selectivity rises to 75 % for catalysts Pd5-Ag95/Al2O3 (h) at 50 % conversion. Fig. 6 shows a unique trend of increasing selectivity with decreasing Pd/Ag ratio. To point out the differences in the selectivity by variation of the Pd/Ag ratio it is essential to investigate the selectivity of the by-products. Fig. 7 shows the selectivity of ethane, 1,3-butadiene, the overall C4 selectivity and the selectivity to C6 compounds. Decreasing of the Pd/Ag ratio in Pd-Ag/Al2O3 egg-shell catalysts effects a decrease of the selectivity of the overall C4 compounds of about 3 % comparing the catalyst Pd100/Al2O3 with Pd5-Ag95/Al2O3, and at the same percentage the selectivity to 1,3-butadiene decreases. The amount of the formed by-products 1butene and 2-butene is constant. An opposite trend is observed for the selectivity of ethane, rising about 1 % with decreasing Pd/Ag ratio, which is in the area of standard deviation. Furthermore, the selectivity of C6 compounds, which is shown in Fig. 7, decreases around 9 % with successive Ag addition from Pd100/Al2O3 to Pd5-Ag95/Al2O3. In summary the gain of ethylene selectivity (Fig. 6) can be interpreted with a reduced formation of oligomers. Additionally, a diminished increase of mass for the Pd-Ag/Al2O3 egg-shell catalysts with lower Pd/Ag ratio is determined due to a reduced oligomer formation. In literature, 1,3butadiene is mentioned as precursor for hydrocarbon and coke formation causing deactivation of the 11


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catalysts in selective hydrogenation of acetylene.

26

The presented work shows C6 compounds, detected at

the gas outlet, as important indicator for the formation of hydrocarbons and coke, affecting long time stability as well as deactivation behavior.

Figure 7. Selectivity to ethane, 1,3-butadiene and C4 compounds (left) and selectivity to C6 compounds as well as the increase of mass of the catalysts, determined by weighing the catalyst pellets before and after each catalytic test, (right) as a function of the Pd percentage of the Pd-Ag/Al2O3 egg-shell catalysts with -1

varying Pd/Ag ratios (T = 30 °C, p = 10 bar, GHSV = 4000 h ).

If each Pd surface atom can serve as an active site for acetylene conversion, then the overall catalytic rate of acetylene conversion should decrease steadily with decreasing number of Pd on the catalytic surface by covering with Ag. With increasing Ag content in Pd-Ag/Al2O3 egg-shell catalysts the total number of Pd surface atoms nPd,surf decreases, determined by CO pulse chemisorption and correlated with CO DRIFTS experiments (Table 2) and the overall acetylene conversion is reduced (Fig. 6). Because the acetylene conversion rate on an Ag only catalyst is negligible compared to a Pd-Ag/Al2O3 catalyst, it is appropriate to express the acetylene conversion rate for Pd-Ag/Al2O3 egg-shell catalysts with respect to nPd,surf, as shown in Fig. 8. The TOF based on the converted amount of acetylene was calculated using the following equation (7): ,-. = /

∆ 1 3∙ ∆1 ∙ 2

%,&'

However, the data displayed in Fig. 8 for the Pd-Ag/Al2O3 egg-shell catalysts show a different behavior to the overall observed conversion, namely, an increase of the TOF with a decrease of the Pd/Ag ratio. The -1

rates per Pd (TOF) are consistent between Pd percentages of 100 at.% to 50 at.% of about 0.2 s , another decrease of the Pd percentage is followed by a significant enhancement of the TOF up to 0.9 s

-1

at 5 at.% Pd. Characterization of the catalysts with CO chemisorption and CO DRIFTS demonstrate a consecutive isolation of large Pd ensembles by successive addition of Ag to Pd-Ag/Al2O3 egg-shell 12



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Page 24 of 32

catalysts at constant Pd loading. The data in Fig. 8 and the characterization results support isolated Pd sites or small Pd ensembles being more active for acetylene conversion compared to Pd surface ensembles containing contiguous Pd atoms. Furthermore, based on the observations we conclude that the Pd catalyzed selective hydrogenation of acetylene on Pd-Ag/Al2O3 egg-shell catalysts is a structure sensitive reaction, because the rates per Pd surface atom are rising with better isolation of Pd ensembles. The observed behavior can be explained by a shift from strongly adsorbed multi-σ acetylene species on Pd-Ag/Al2O3 egg-shell catalysts with low Ag content and large Pd ensembles to a weakly bonded πspecies on isolated Pd sites at Pd-Ag/Al2O3 egg-shell catalysts with a high Ag content. The change of adsorbed acetylene species leads to a decrease of the total amount of adsorbed acetylene per Pd surface 19

atom.

Additionally the formation rate of ethylene through weakly bonded acetylene π-species is much

faster than through a strongly bonded σ-species.

19

These two facts explain the observed increase of TOF

due to an isolation of large Pd ensembles.

Figure 8. Turnover-frequency (TOF) of the Pd-Ag/Al2O3 egg-shell catalysts with varying Pd/Ag ratios (T = -1

30 °C, p = 10 bar, GHSV = 16000 h ).

13


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The described findings on industrial relevant Pd-Ag/Al2O3 egg-shell catalysts are in good agreement with the observations of Y. Zhang et al.

19

on a 1.85 wt.% Pd/SiO2 catalysts promoted with Ag and Au. The

authors suggest the promoting effect is only geometric in nature and not electronic. The results are confirmed by kinetic experiments shows a much lower activation energy and a less negative reaction order in acetylene pressure on Pd-Ag/SiO2 in comparison to Pd/SiO2. The influence on C4 and C6 byproduct formation is not described in detail in the literature but playing an important role in industrial application. 1,3-butadiene is known to be the precursor for green oil formation, which leads to the deactivation of catalysts used in the selective hydrogenation of acetylene.

26, 52, 53

Decrease of the Pd

percentage shows an obvious reduction of selectivity to 1,3-butadiene, C4 and C6 and compounds (Fig. 7), as described above. This observation can be explained again by a consecutive isolation of large Pd ensembles by successive addition of Ag to Pd-Ag/Al2O3 egg-shell catalysts, due to 1,3-butadiene formation, coupling to higher hydrocarbons and hydrogenation proceed at least on two adsorption sites. Mechanistic study of 1,3-butadiene formation over Pd-based catalysts using DFT-calculations by Yang et al.

54

confirm these results. Additionally the formation rate of 1,3-butadiene is slightly lowered on a Pd-Ag

surface than on a Pd only surface. Furthermore Fig. 7 shows a marginal increase of ethane selectivity with decreasing Pd percentage, giving evidence, the hydrogenation of ethylene takes place on isolated Pd sites or small Pd ensembles, too. Addition of Ag to Pd/Al2O3 egg-shell catalysts affects a decrease of hydrocarbon and coke formation and fouling, as described above. To connect deactivation behavior with these observations, 100 h long-term tests were carried out with Pd100/Al2O3 and Pd5-Ag95/Al2O3. The long-term tests were conducted at an acetylene conversion of about 70 %, because this correlates with the conversions within the first catalyst 3

bed of an industrial reactor where the fastest deactivation of the catalyst is observed. Therefore the reaction temperature has to be adjusted to 27 °C for Pd100/Al2O3 and 45 °C for Pd5-Ag95/Al2O3. The conversion behavior during 100 h long-term test for the two catalysts is shown in Figure 9. The conversion decreases linearly with about 0.07 % per hour for Pd5-Ag95/Al2O3, resulting in a deactivation of 6.8 % over 100 h. However, for the Ag free Pd100/Al2O3 egg-shell catalyst the conversion increases after 10 h run-in period from 71 % to about 76 %. After 36 h TOS the conversion of the Pd100/Al2O3 egg-shell catalyst starts to decrease to 59 % after 100 h with a rate of about 0.24 % per hour. The fast deactivation of Pd100/Al2O3 can be explained with the amount of formed hydrocarbons and coke, which is much lower on Pd5Ag95/Al2O3 with 2.5 wt.-% than on Pd100/Al2O3 with 18.1 wt.-%. An indicator for these findings is the combination of the selectivity to C4 and C6 compounds as described above. The selectivity to the target product ethylene declines of 5.6 % for Pd5-Ag95/Al2O3 and of 35.1 % for Pd100/Al2O3. This observation is affected by an increasing selectivity to ethane, rising much faster for Pd100/Al2O3 than for Pd5-Ag95/Al2O3 and has the same trend like ethylene selectivity decreases. The proceeding formation of hydrocarbons and coke reduce the effective diffusion coefficient followed by a rising mass transfer resistance, which leads to an increased consecutive hydrogenation of ethylene to ethane. Asplund

26

reported about an

decrease of the effective diffusion of about 1/10 for a Pd/Al2O3 catalyst after 100 h TOS. Furthermore the deposited green oil serves as hydrogen reservoir especially for the consecutive hydrogenation of ethylene 14



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to ethane.

28

Page 26 of 32

The presented results demonstrate a higher long-term stability of the Pd/Al2O3 egg-shell

catalyst added with Ag, and thus the time between regeneration cycles can be reduced and the economy is enhanced because of higher ethylene selectivity and yield. The deactivation is caused by fouling of coke and hydrocarbons on the catalytic active surface.

Figure 9. Conversions (left) and selectivity to ethylene (right) during 100 h long-term test for catalyst -1

Pd100/Al2O3 (T = 27 °C ) and Pd5-Ag95/Al2O3 (T = 45 °C), (p = 10 bar, GHSV = 4000 h ).

4. Conclusion The sequential addition of Ag to prepared Pd-Ag/Al2O3 egg-shell catalysts was investigated by CO pulse chemisorption and CO DRIFTS measurements and a bimetallic character was detected. A decrease of the total number of Pd surface atoms and a consecutive compensation of large Pd ensembles as well as an isolation of Pd surface atoms was observed by successive addition of Ag to Pd-Ag/Al2O3 egg-shell catalysts. TPR measurements indicate an interaction of Pd and Ag, due to a shift of Pd reduction peak and an absence of Pd hydride formation. The influence of activity and selectivity in selective hydrogenation of acetylene was studied. The enhanced TOF of acetylene conversion for low Pd/Ag ratios relative to high Pd/Ag ratios demonstrate that the critical reaction sites for acetylene conversion consist of small Pd ensembles, implicating a structure sensitive reaction due to a shift of adsorption mode of acetylene. Furthermore the reduction of large Pd ensembles decreases the amount of formed by-products like C4 and C6 compounds, which are crucial for the formation of hydrocarbons and coke deposited on the catalyst surface and causing deactivation of the catalyst. Long-term test of 100 h confirm these observations by a decrease in activity on Pd100/Al2O3 much faster as on Pd5-Ag95/Al2O3. For catalytic testing in laboratory scale the selectivity to C6 compounds is a fundamental indicator for long-term stability of palladium containing catalysts in selective hydrogenation of acetylene.

15


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Supporting information available TEM images and crystallite size distribution are available in the supporting information. This information is available free of charge via the Internet at http: //pubs.acs.org.

16



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Figure captions

Scheme 1. Reaction network of the selective hydrogenation of acetylene.

Figure 1. A cross section of a Pd-Ag/Al2O3 egg-shell catalyst fresh (left) and used (right) after 100 h TOS.

Figure 2. Pd- and Ag- concentration in dependence of the pellet diameter for the catalysts Pd100/Al2O3 (left) and Pd80-Ag20/Al2O3 (right), measured by EPMA.

Figure 3. Example of investigation by STEM a), EELS b) and EDX c) for Pd70-Ag30/Al2O3.to prove bimetallic Pd-Ag particles.

Figure 4. TPR profiles of (b) Pd100/Al2O3, (d) Ag100/Al2O3 and (a) Pd70-Ag30/Al2O3 calcined at 500 °C and (c) Pd100/Al2O3 calcined at 400 °C.

Figure 5. DRIFT spectra of (a) Pd100/Al2O3, (b) Pd95-Ag5/Al2O3, (c) Pd80-Ag20/Al2O3, (d) Pd70-Ag30/Al2O3, (e) Pd50-Ag50/Al2O3, (f) Pd30-Ag70/Al2O3, (g) Pd20-Ag80/Al2O3 and (h) Pd5-Ag95/Al2O3 after 1 h CO exposure at 45 °C.

Figure 6. Conversion plotted vs. the modified residence time τmod (left) and Sethylene-Xacetylene diagram (right) of the Pd-Ag/Al2O3 egg-shell catalysts with varying Pd/Ag ratios (a) Pd100/Al2O3, (b) Pd95-Ag5/Al2O3, (c) Pd80-Ag20/Al2O3, (d) Pd70-Ag30/Al2O3, (e) Pd50-Ag50/Al2O3, (f) Pd30-Ag70/Al2O3, (g) Pd20-Ag80/Al2O3 and (h) Pd5-Ag95/Al2O3 (T = 30 °C, p = 10 bar).

Figure 7. Selectivity to ethane, 1,3-butadiene and C4 compounds (left) and selectivity to C6 compounds as well as the increase of mass of the catalysts, determined by weighing the catalyst pellets before and after each catalytic test, (right) as a function of the Pd percentage of the Pd-Ag/Al2O3 egg-shell catalysts with -1

varying Pd/Ag ratios (T = 30 °C, p = 10 bar, GHSV = 4000 h ).

Figure 8. Turnover-frequency (TOF) of the Pd-Ag/Al2O3 egg-shell catalysts with varying Pd/Ag ratios (T = -1

30 °C, p = 10 bar, GHSV = 16000 h ).

Figure 9. Conversions (left) and selectivity to ethylene (right) during 100 h long-term test for catalyst -1

Pd100/Al2O3 (T = 27 °C ) and Pd5-Ag95/Al2O3 (T = 45 °C), (p = 10 bar, GHSV = 4000 h ). 20



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