Electron-deficient palladium clusters and bifunctional sites in zeolites

283

Catalysis Today, 12 (1992) 283-295

Elsevier Science Publishers B.V., Amster&m

ELECI’RON-DEFICIENT

PALLADIUhf

CLUSTERS AND BIFUNCTIONAL

SITES IN

ZEOLITES

W.M.H. SACHTIER

AND AYU. STAKHEEV

V.N. Ipatieff Laboratory, Department 60208 USA

Zeolite supported

of chemistry, Northwestern

metals are important

particles are largely determined

Univemity, Evanston, II,

catalysts; the c~~e~ti~

of the metal

by the interaction with the support. The zeolite matrix not

onlyimposes steric constraints for reacting molecules (shape-selective catalysis) and provides acid sites @functional

catalysis), but it also affects the electronic properties

Boudart and other researchers

of the metal.

have presented a strong case for such effects in Pt/Y [1,2].

They found that the catalytic activity of 1 nm Pt chrsters in Y zeolite for neopentane isomerization and hydrogenolysis is 40 times higher than that of Pt/siO, and Pt/AlsOs. Boudart ascriied

this increase in activity to an electron transfer from Pt to the zeolite support and

introduced the term “electron-deficiency”

for such Pt clusters. He suggested

that, as a result

of this transfer, Pt behaves like Ir, its left hand neighbour in the Periodic Table and a metal exceeding Pt in catalytic activity for neopentane

conversion

by more than two orders of

magnitude. Electron-deficient

metal particles display indeed some intriguing catalytic properties.

They exhibit an enhanced catalytic activity towards hy~ogenation

[3-S], hy~geno~~

[3,4,7-g] and a high resistance to sulfur poisoning [lO-121. Recently it was also found that Pd particles in Y zeolite exhiiit unusual selectivity in methylcyclopentane

conversion [13].

The modification of electronic and catalytic properties of metal clusters in zeolites has been critically reviewed by Gallezot [14,15]. He showed that in addition to the changes in electronic structure of small metal particles due to intrinsic size effects, the electronic structure of the metal is modified by the particle environment,

e.g. electron transfer from metal to

electron acceptor sites in the zeolite lattice. He considered the following centres as potential electron

acceptors

: 1) 3r#nsted

Recently, the interaction

acid sites; 2) Lewis acid sites; 3) multivalent

of an Ir, cluster with a Mgs+ ion has been ~eo~ti~

by van Santen et. al. using a nonempirical

~20-~1/92/$13.~

Hartree-Fock-Slater-LCAO

0 1992 Ekvier Science Publishers B.V. All rigbta resewed

cations. eclats

method 1161. It was

shown that the Mg?+ cation polarizes the Ir4 cluster such that the negative charge accumulates close to the Me

ion. This leads to an increase in the adsorption energy of Hs. These authors

suggest that the polarization activity of metal-zeolite

of metal clusters will be responsible for the enhanced catalytic

systems in conversion of saturated hydrocarbons

(vide infra).

In later studies Gallezot et al. found correlations between zeolite Br@sted acidity and the electrophilic Independent

character

investigation

electron-deficient

of Pt particles of Blackmond

using X-ray absorption

and Goodwin

also provided

spectroscopy evidence

[17’J.

that the

character of the metal increases with the acidity of the support [18]. In a

more recent investigation by Wang et al. on the chemisorption properties of Ru supported on a series of zeolites of different acidities, a significant suppression in I&-chemisorption observed and ascrii

to electron transfer between metal and acid sites [19]. Interestingly,

only catalysts prepared vapour-impregnation

has been

via ion-exchange

exhiiit

this suppression.

Samples prepared

of Rus(CO)i2 did not exhibit a change the chemisorption

via

properties,

probably as a result of the absence of acidic hydroxyl groups in close vicinity to metal particles. Resistance to sulfur poisoning also increases with Br&sted a deahnninated groups increases

acidity. It was found that

Pd/NaHY zeolite shows enhanced resistance [14] because the acidity of OH upon partial dealumination.

Electron

deficiency was confirmed

by IR

spectroscopy of chemisorbed CO. Exchange of the protons by Na+ in a NaOH solution almost entirely suppressed the electron deficiency of the metal. Evidence for bond formation between Pd and zeolite protons. 1. In&ared Spectroscopy of Chemisorbed CO Direct evidence that zeolite Br#nsted acid sites, i.e. protons, are responsrble for the formation of electron-deficient

metal particles has been found recently by Sheu et al. [20]. It

was shown that CO admission to Pd/NaHY

reduced below 35O“C leads to formation

of

Pd13(CO)n carbonyl clusters which perfectly fit inside supercages. Purging in inert gas at room temperature

gives rise to easy release of CO. This is incompatible

with thermal desorption

because the heat of adsorption of CO on Pd is about 30 kcal/mol[21], and since the activation energy of desorption must be equal to or larger than this value, thermal desorption should be very slow. It was also found that simultaneously

at RT

with CO removal the intensity of

the O-H band at 3647 cm-’ decreases (Fig.1). These processes were attriiuted

to a substitution

of CO ligands by zeolite protons. Taking into account that the bond between H and Pd is strong, it is conceivable that substitution of the CO ligands by protons might be energetically favourable. This substitution results in the appearance

of a positive charge on Pd clusters;

285

560

Fig. 1. FTIR

indeed

:

'0

spectra of hydroxyl groups in PdJCO)JNaHY

the characteristic

wavenumbers.

’ 3700 3490 3&o UAYENUM6ER

frequencies

of the remaining

after various purging times.

CO ligands shift toward higher

This substitution was found to be completely reversible: reintroduction

of CO

restores O-H and C-O band intensities and positions. If the concept of Pd-H bond formation is correct, an increase of the proton concentration

in the vicinity of the metal cluster should

increase the positive charge on the metal. Experiments

with Pd/CaY and Pd/MgY have

confirmed this model. In the above mentioned PdjNaHY catalyst, Pd ions are located in small cages after SOOT calcination

and they migrate to the supercages during reduction

behind most of the protons formed during this process [22,23]. For PdKaY

and Pd/MgY

catalysts Pd’+ ions remain in supercages after calcination, because a location of Me C!a’+ in small cages is preferred

leaving

and

due to the high charge density in the small cages. The

location of Pd2’ ions in supercages has been confhmed by TPR data: the presence of Mgs

and Ca” facilitates their reduction and results in a downward shift by 70°C of the ‘I’PR peak for Pd2+ reduction [24]. For these samples, the proton’concentration

around the Pd chtsters

is significantly higher, because all protons formed during reduction are located in the same supercage as the Pd cluster. FfIR

of chemisorbed

CO shovved that the resulting positive

charge on the metal is higher: the CO stretching frequency of linear adsorbed CO is shifted towards higher wavenumbers

(2128 cm-’ for Pd/MgHY and 2132 cm-l for Pd./CaHY) as

compared to Pd/NaHY (2120 cm-‘). This shift cannot be attriluted

to CO adsorbed on Pd+

ions, since the band of the bridging CO is also shifted Tom 1898 cm-’ for Pd/NaHY to 1910 cm-’ for Pd/MgHY and 1913 cm-’ for Pd/C!aHY. TPR data congrm complete reduction of Pd/Y samples at the temperature

used (35OT) [22].

2. X-ray Photoelectron Spectroscopy of Pd Clusters XPS data confirm that the electron-deficiency

the concentration

of metal particles in zeolites depends on

and location of Br&sted acid sites [25]. Experiments with Pd/NaHY, before

and after neutralization

with NaOH or NHs, and with Pd/MgHY or Pd/HY show that the

binding energy (BE) shift for Pd 3&n

due to electron transfer between Pd cluster and

Br@sted acid sites amounts to about 0.4 eV. This result was obtained by subtracting the BE shift due to intrinsic size effects from the total BE shift of Pd in Pd/NaHY. For Pd/MgHY and Pd/HY the positive charge is much higher due to the higher proton concentration

in the

supercages; in these zeolites the BE is shifted by 0.8 eV (Fig.2). Two explanations proton concentration:

can be considered for this rise in positive charge with increasing

(1) an increase of the number of clusters interacting with a proton; (2)

an increase of the number of protons interacting with each cluster, leading to an increased charge on each cluster. It is of relevance that the width of the Pd 3ds,, line does not change significantly while the Pd 3d,, BE gradually increases in the order: Pd/NaHY_,. < Pd/MgHY - PdkIY.

This shows that the distriiution

< Pd/NaHY

of the charge of the clusters remains

relatively uniform while the average charge of the cluster gradually increases. Therefore, we can conclude that Pd clusters are capable of interacting with more than one proton; the BE increases because more protons interact with a given metal cluster.

If only the number of

clusters that interact with a proton would increase, this would result in a broadening

(or even

splitting) of the Pd 3d,, binding energy peak, because in this case two types of Pd species would coexist: Pd6+ and Pd”. The absence of line broadening

or splitting also provides strong evidence against

formation of Pd+ ions, as coexistence of Pd“ and Pd+ should result in broader peaks than existence of Pd“ only, as Kevan et.al. showed [26]. Such broadening

would be especially

287

Binding

Energy,

ev

Fig. 2. XPS spectra of Pd 3dSD line for Pd/NaHY, correspond to FWHH of Pd 3dSn line (eV).

pronounced

at approximately

equal concentrations

and Pd/MgHY. Numbers

in brackets

of both species.

The Pd Auger parameter, which is sensitive to the polarizability of neighboring atoms, gradually decreases in the sequence Pd/NaHYmm. >Pd/NaHY>Pd/MgHY>Pd/HY.This trend reflects the increase of the number of nonpolarizable

protons attached to the metal

cluster.

Experiments with hydrogen isotope exchange on Pd/HY and Pd/NaHY give additional evidence for the interaction between Pd clusters and zeolite protons, which are formed either

288

due to decomposition

of NH4+ or during reduction [271. The protons which are in intimate

contact with Pd clusters are exchanged against gaseous D, with the same rate as H atoms adsorbed in Pd. The number of these easily exchangeable protons is higher for Pd/hfgHY due to their higher concentration

in close proximity to Pd clusters.

a

Fig. 3. Atomistic model of [p&-H.J’ adduct formation: a) without interaction; b) complete detachment of the proton; c) formation of 0...H.-Pd,,, bonds. Atomistic Model of [pd,,,-]‘+

Formation.

All data mentioned so far provide strong evidence that zeolite protons are responsible for the formation of electron-deficient

metal particles. An atomistic model is the metal-proton

adduct model proposed by Sachtier et al. [&?8,29]. In view of the high metal-hydrogen

bond

strength it is conceivable that protons react with metal particles forming Pd-H bonds. In the resulting

[p%-H.J ‘+ adduct the positive charge will not be localized on the protons.

atoms of the cluster sharing the positive charge are thus “electron-deficient”

The Pd

using Boudart’s

289

terminology [1,2]. In the limit a proton can be imagined completely detached from the oxygen ions of the zeolie,

however it is more likely that protons will occupy bridging positions

between metal particle and zeolite wall (Fig.3). Using Pd/NaY as an example, the formation of positively charged metal-proton adducts can be described by: n~d2++2~~+nH2-~Pdo~+~H~~ P@s + z H-o,,

--

+ z 0-,,

[p4iH.J=+

As shown above the number z of the protons interacting with the metal cluster depends on the local proton concentration in its close proximity. On the basis of this model it is possrbie to relate the degree of electron-deficiency of metal clusters and their catalytic properties to parameters such as Br#nsted acidity, metal particle location, and the presence of multivalent cations. ~~~d~

Efhts

Rtmaiting fimu m-HJ”

lhmati

Formation of pd,,,-H.$+ c8n provide a rationalization of experimental observations, including the following: 1. anchoring of metal clusters; 2 unusually high activity in metal catalysiq 3. unusually high activity in bifunctional (metal + acid) catalysis. 1.Anchoring of the metal particles

At low temperature coalescence of zeolite supported metal particles is restricted by the size of the cage window. Particles tbat are smaller than the cage apertures can also be immobilized by electrostatic interactions with negatively charged oxygen ions on zeolite walls. This type of stabilization requires that the metal cluster carries a positive charge or that it is strongly polarized. EXAFS data on Pd/NaHY zeolite show that after reduction at 35ooC the average coordination number for Pd particles is about 4.0, suggesting an average cluster size of six atoms as a regular octahedron (Pd-Pd interatomic distance 2.68 A f 0.01 A) [30]. The effective diameter of these chwers is about 6.5-7.0 4 their stability is remarkable: metal dispersion does not change significarulyup to a reduction temperature of4OO”C.Taking into account that the diameter of a supercage wind& in Y zeolite at room temperature is about 7.4 A [31] and that Pd,, clusters can easily traverse supercage windows at 2WC [32], one might expect easy migration and coalescence of the P& cluster in Y zeolite at higher reduction temperatures.

It has been proposed that a major cause of the surprising immobility of the clusters is their interaction with protons resulting in [p&-I-I$+ bridging 0...H-.Pd,

bonds. Admission

of CO at room temperature

interaction

between

antibonding

orbitals of CO. Consequently,

clusters. EXAlS

clusters

adducts which are kept in their position by the

and protons

due to electron

lowers the extent of

withdrawal

into empty x*-

CO adsorption induces rapid coalescence of Pd,

data show that in zeolite Y this process leads to rapid formation

Pd,,(CO)y clusters with a Pd-Pd coordination

of

number of - 6. The diameter of these clusters

exceeds that of the supercage windows, so that further migration and coalescence is inhibited by steric constraints [32]. 2. Catalytic Activity of Electron-Defldent Clusters Literature

evidence for high catalytic activity of oxide supported

metal particles is abundant. hydrogenation

electron-deficient

The activity per exposed metal atom of Pd/&O,

in benzene

e.g. is three times higher then that of PdBiO, [33]. It was also shown that a

Pd/AlsOs catalyst exhibits very high activity in hydrogenolysis after treatment

in helium at

6OO“C [34]. The activity of these catalysts has been ascrii

of Pd+ ions

to the presence

stabilized either by strong interaction with coordinatively unsaturated

A13+ ions or in cation

vacancies of the support surface. Prins et al. [35,36] have shown that Pt+ and Rh2’ ions can act as “anchors” for highly dispersed metal particles. Charge sharing will induce a positive charge on such metal particles, as is indicated

by several methods,

including

FTIR of

chemisorbed CO, ESR and others [371. Enhanced catalytic activity of electron-deficient has been reported for benzene hydrogenation

Pd in zeolites in comparison to Pd/SiO,

[7,8]. The observed activity sequences correlate

with zeolite acidity: NaX < NaY < CaY < MgY < CeY - HY I Lay. Catalytic superactivity of Pt/Y zeolite in hydrogenolysis has been reported by Boudart et al. [3,4]. Very high activity of Pd/Y in neopentane

conversion has been reported recently [lO,ll]. However, the nature

of the electron-deficiency

in these cases differs considerably from that in Pd/A120s. It was

shown that for Pd/NaHY concentration

the activity in hydrogenolysis

(Fig 4). A sample prepared

followed by calcination

is changing

with the proton

by ion exchange of Pd(NI-QJ2’

at 5OOT loses all NH, ligands. Upon subsequent

ions in Nay, reduction,

two

protons are formed per Pd atom. Such samples displayed a catalytic activity per exposed Pd atom two orders of magnitude higher than PdBiO,.

This contrasts with similar samples which

had been calcined only up to 250°C to retain part of the ammine ligands as Pd(NH3)22+ ions. Upon reduction

of those samples a signiticant fraction of the protons are neutralized

by

released IQ-& Iigands. Indeed, these samples display a much lower catalytic activity, though

291

-5

-9

18

,

0.18

I

0.2 1 (E-2)

0.20

0.19 1/T(K)

Fig. 4. Arrhenius plots for neopentane conversion over:() 2,4, and 7 wt% Pd/NaHY reduced at 500 “C; (---) 54, and 7 wt% Pd/NaY reduced at 250 “C; (-) 0.7 wt% Pd/SiO,.

still somewhat higher than Pd/SiO,.

These results support the view that formation of [p&-

H.$+ adducts is responsible for the unusually high catalytic activity of the sample which was calcined at 500°C. This conclusion is confirmed by neutralization

of the zeolite protons with

NaOH which lowers the catalytic activity of the metal function to the level of Pd/siO,.

3. Formation of Bifhnctiond

Sites

Certain catalytic reactions

are known to require

‘bifunctional

catalysts”, i.e. system

which displays both metal and acid sites. The bifunctional model proposed by Mills et al. [38] for catalytic reforming of n-hexane assumes formation of an olefinic intermediate

on a metal

site, its migration to an acid site, where a carbenium ion is formed. The model proposed by Kouwenhoven

[39] assumes that direct hydride-ion transfer between paraffin and carbenium

ion results in the formation of a new carbenium

ion without an olefinic intermediate.

Pt/HY Chow et al. [40] showed that both reaction involving dehydrogenation

For

paths are used. However, the route

on metal centers and isomerization

of carbenium ions on acid sites

was found considerably more effective. The concept of the [pk-HJ+ The ummpt of the [Pd&IJy

adduct suggests an alternative adduct suggesti an alternative

reaction path, without reaction path, without

292

shuttling of reaction intermediates

between metal and acid sites. The adduct is a hybrid which

should be able to act as metal and as acid site, so that completion of a “biftmctional” reaction should be possible during one residence of the molecule on such a hybrid site.

s0

20

T

--c

(d)Pd/NaY

X

-+

(c)HY+Pd/NaY-neutr

g

16

-is F

12

$

P Y” 3 A? k!

4

0

0

20

40

60

Time on Stream

80

100

(min)

Fig. 5. Ring enlargement of MCP over PdEIY, Pd/NaHY, Pd/NaY_ti., mixture of HY + PdlNaY,,*.

An appropriate

120

HY and physical

probe reaction is the catalytic conversion of methylcyclopentane.

It is

well established that ring opening (RO) occurs on metal sites, but for ring enlargement

(RR)

the presence of both metal and acid functions is required. Recently Bai and Sachtler [41] found that Pd/HY catalysts are dramatically more active in RE than a physical mixture of HY and neutralized Pd/NaY (Fig. 5). The concentrations

of either type of sites were approximately

equal in these two samples; however, the initial RE rate of MCP on Pd/HY was 20 times greater than on the physical mixture. Considering the prior evidence for the existence of [pd,,H$’

adducts, it was proposed that on this catalyst the bifunctional reaction occurs during one

residence on a [p&-H$+

adduct (Fig. 6). In accordance with this mechanism the MCP

c-

+ + [Pd,.,-H1+ -

H\ dn

+

90

0 + “/

[Pd,,-HI+ t

293

H\

2 -

H’

\ [PdnHf

+

0 ()

t 3H2

Fig. 6. Mechanistic model for MCP ring enlargement over Ipg-I+$‘. molecule chemisorbed on the charged [P&-HJ’

adduct forms a polarized alkyl group with

a carbenimn ion as its limiting structure. Isomerization of the adsorbed species results in the formation of cyclohexane as a primary product.

It is subsequently either desorbed or

dehydrogenated to benzene. Increase of the reaction temperature or a decrease of the H, partial pressure will shift the surface equilibrium to benzene. Temperature-programmed surface reaction experiments support this model; they also suggest that for the RO reaction Pd clusters are more active than [p%-H.J+ adducts [41]. It is interesting that Pd/HY is much more active for RE than Pd/NaHY, although both catalysts contain Pd clusters and protons. These catalysts differ, however, in the proton concentrations inside the supercages, where the Pd clusters are located. As was shown above on the basis of XPS results and FTIR data of chemisorbed CO, the local concentration of protons in supercages is decisive for the z value of the adduct [pd,,-I-&j”‘. It thus seems that the RE reaction requires a higher positive charge on the [pk-I-I$+ adducts than, for instance, the hydrogenolysis of neopentane for which Pd/IW-IY is a very active catalyst. A need for a high positive charge in RE would be consistent with the general assumption that isomerization

294

of the five-membered

to the six-membered ring requires fdrmation of a “carbenium ion-like”

intermediate.

In this context it is of interest that for Pd/Y catalysts comaining different charge

compensating

cations the activity for RE decreases in the order: Pd/LiY > Pd/NaY > Pd/KY

[42]. This sequence has been rationalized in terms of supposedly increasing electron donation to Pd clusters [43]. It is, however, difficult to see how alkali ions with high second ionization potential could act as the effective electron donor. An alternative model would assume that the formation of proton adducts, which act as hybrid sites, is affected by the size of the ions occupying the same cage. Another model considers the polarization

of metal clusters due to

the electrostatic field of adjacent alkali ions [16].

C!oncl~

Remark

Chemical interaction substantiated

of protons with transition

by a number of independent

metal clusters in zeolites has been

chemical and physical techniques which show that

protons compete with other molecules for adsorption

sites on Pd clusters. The palladmm-

proton adduct provides a possible rationale for: 1)

anchoring of Pd clusters in zeolites;

2)

enhanced activity for metal catalyzed reactions by “electron-deficient”

3)

enhanced

activity for bifunctional

particles;

reactions on hybrid sites acting simultaneously

as

metal and as acid sites.

Aclmowiedgement Financial support of the National Science Foundation, grant-in-aid by the Mobil Corporation

contract CI’S-8911184, and a

are gratefully acknowledged.

References 1 2 3 4 5 6 7 8 9 10

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