Layered double hydroxides as solid base catalysts and catalyst precursors

I. Kiricsi, G. Pdl-BorbEly, J.B. Nagy, H.G. Karge (Editors) Porous Materials in Environmentally Friendly Processes Studies in Surface Science and Catalysis, Vol. 125 9 1999 Elsevier Science B.V. All rights reserved.

329

Layered double hydroxides as solid base catalysts and catalyst precursors Didier Tichit and Francois Fajula Laboratoire de Mat6fiaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 CNRS. ENSCM, 8, rue de l'Ecole Normale, 34296 Montpellier CEDEX 5, France Examples on the use of layered double hydroxides as solid base catalysts, catalyst precursors and supports are presented. The factors influencing the nature and strength of the basic sites are briefly reviewed and the discussion focusses on the application of LDHs and mixed oxides for the aldol condensation of acetone, the selective hydrogenation of nitriles and the oxidation of mercaptans.

INTRODUCTION The last decade has seen a strong incentive towards the development of solid base catalysts for two main reasons. In the one hand, this family of solids constitutes a rather virgin field for basic research as compared for instance with solid acids or supported metals, and, in the other hand, the more stringent environmental regulations pertaining to pollutant disposal force the search for alternative cost-effective solutions for substituting liquid bases in bulk chemicals production. Systems which have received most attention are the alkaline earth and mixed oxides, modified carbons, resins, ion-exchanged zeolites, supported alkali metals and amides, hydrotalcites, oxynitrides. Among those, hydrotalcites and more generally speaking layered double hydroxide materials (LDHs), and the mixed oxides produced by their calcination seem to be the most promising at present for practical applications due to their easiness of preparation and handling (1,2). LDHs have a brucite-like (Mg(OH)2) structure which consists of stacked hydroxyl layers of edge-sharing octahedra and of interlamellar space separating the layers. The substitution of a divalent metal cation for a tdvalent one in the neutral layer generates an excess of positive charge which is counterbalanced by exchangeable anions located, as water molecules, in the interlayer space. LDHs include a variety of compositions with general formula: (M2+l.x M3+x (OHh)(A"')~,n, z H20 where M 2+ ions are Mg, Ni, Zn, Co, Fe, Cu..., M 3~ are Al, Fe or Cr... and An are CO3, SO4, NOa, halogen, hydroxide. LDHs are readily prepared by co-precipitation of the suitable reagents but may be also prepared by controlled hydration of commercially available mixed oxides. Although the LDH by itself develop basic character and catalytic activity after adequate exchange of the synthesis anions, as we shall see below, most studies on the use of LDHs in base catalysis have been performed on the calcined materials. Upon heating LDHs at temperatures above c a 400~ a high surface area mixed oxide is obtained featuring active surface sites including hydroxyl groups and O2-M~§ acid-base pairs. Due to the variety of compositions that can be achieved and to the very strong impact of the thermal history of the samples on the nature, distribution, and strength of the sites, the catalytic properties of LDH-derived mixed oxides may significantly vary in terms of activity

330 but also selectivity depending on the preparation conditions. Moreover, the incorporation of reducible cations in the inorganic framework and the possibility of interacalating anions with different sizes and functions in the interlayer space offers unique perspectives for the design of new base/redox bifunetional catalysts. This contribution overviews some recent developments on the use of LDHs as precursors for the preparation of versatile catalysts, including the selective synthesis of isophorone or diacetone alcohol by condensation of acetone, the selective hydrogenation of nitriles and the oxidation of mereaptans into disulfides. Mixed oxides as solid base catalysts

LDHs or their mixed oxides derivatives are efficient catalysts for base catalysed reactions generally performed with alkaline or alkaline earth oxides, anion exchange resins or in homogeneous catalysis with alkali hydroxides. The catalytic properties of the mixed oxides obtained by thermal decomposition are influenced by several parameters of LDHs precursors, namely the chemical composition, the calcination temperature, the synthesis procedure. Indeed the former, could be varied through several parameters i.e. the nature of the M ~+ and M 3+ cations in the layers, the M2+/M 3+ ratio and the nature of the compensating anions (3). A 2+ 2+ 2+ wide variety of mixed oxides obtained from LDHs containing in the layers Mg , Fe , Co , Ni 2+ or Zn2+ and AI3+, Cr3+ or Fe3+ cations and in the interlayer space CO32, SO4 2-, frO42, halides or NO 3" have been compared in the aldol condensation of acetone (3) and the cross condensation between formaldehyde and acetone (4). The mixed oxides issuing from the 2+ 3+ 2decomposition of the Mg -AI -CO 3 LDH were the most active and showed typical basic activity. Only vaporizable inorganic (CO32, NO3 ) or organic anions (acetate, oxalate...) lead 2to active catalysts, and CO 3 was prefered (3,4). However specific behaviours were noted for

$042 or CrO/" for example (3.5), grafted to the layers thus blocking the active sites, or for CI whose diffusion in the structure upon heating, enhances the acid character (6,7). Mg 2+ and AI3+ containing mixed oxides, the most widely used, successfully catalyse aldol and Knoevenagel condensations (3,4,8,9), synthesis of chalcones (9,10), polymerization of 13propiolactone (11 ), H-D exchange of acetone and toluene (3), alkylations of phenol (12-14). Qualitative analysis of the acido-basic properties of different types of LDHs by the color change of indicators, reveal pKa in the range of 6-12.2 (15). However, according to the above results on the reactivity, the Mg/AI mixed oxides exhibit the stronger basicity among mixed oxides with sites in the range of 4.5
331 with different nature and strength, whose distribution depends on the molar fraction in aluminium and the calcination temperature of the precursors LDHs. Extensive studies by IR spectrocopy of adsorbed CO: and by XPS measurements evidenced that the inclusion of AI3+ greatly modifies the nature of sites prevailing on MgO. In this later three types of sites with decreasing basic strength have been recognized for long time: O 2, O in the vicinity of OH groups, and hydroxyl groups (19). There is a general agreement about the presence on the Mg-AI mixed oxides of sites with high, medium and low basic strength respectively, due to isolated O 2-, Mg2+.O2- or AI3+-O2" pairs and OH groups. Mg2+-O2 pairs more likely contribute to the medium basicity than the AI3+-O2 pairs (19,20). Mg 2+ and AI3+ contribute for Lewis acid sites (18). TPD (16) and mierocalorimetric adsorption studies of CO 2 (18,21) as well as t3CO~/~2CO2 isotopic exchange experiments (20) show that the density of basic sites decreases and that the sites with higher strength are suppressed on the Mg-AI mixed oxides compared to MgO. The mixed oxides also present lower acidity than AI:O 3 (21).

A

3.o I

~ ~

2.5

Cak:tmlM)n Tm'npm'atum -'[]-- 673K " 0 - 773K

i

Figure 1" Amount of irreversibly held CO2 as a function of chemical composition (20)

.0 1.5 1.0

"~.

0.5 0.0 , 0,0

9

, 02

9

1

0.4

x

9

i

0.6

'

9

i

0.8

-

1.0

= AI/(AI + Mg)

These phenomena were explained by a complex reorganization taking place on the mixed oxide surface. Several models for these structural evolutions were suggested, accounting for the concurrent modifications of basieities and reaetivities. Derouane et al (22) emphasized on the geometrical arrangement of the crystallographic planes exposed at the surface. Migration of AI in the MgO structure, evidenced by a decrease of the lattice parameter with concurrent apparition of tetrahedrally coordinated AI, led them to consider that AI3+ are incorporated in the rock-salt structure, creating Frenkel defects. Therefore in this structure, replacement of three Mg 2+ by two AI3+ creates one cationic vacancy. The surface acting as a sink for the vacancies, one Mg 2+ will be removed inducing an excess of O2 sites. They assume that these sites are more easily aecomodated by the oxygen terminated (111) planes than by the (100) planes exposing metal cations and oxygen. It comes out that the (111) planes are stabilized either by an increase of the AI amount or by the temperature, both inducing migration of AI. As a result the basieity increases. For Al-rieher samples or for too high calcination

332 temperature, segregation ofMgAl20 4 phase occurs leading to a drastic decrease of basicity. A slightly different mechanism was proposed by Fishel et al (23) also based on the stabilization of the (111) planes. Considering that the stabilizing effect of AI should inhibit the formation of defects on the surface, they assume that they were more likely created by incorporation of 02. provided by the decomposition of CO32 9CO32" are associated to AI3§ and basicity should increase with AI content. The model proposed by Di Cosimo r al (20) was mainly based on XPS measurements of oxygen, aluminium and magnesium surface atomic fractions compared to that of the bulk obtained by elemental analysis on mixed oxides with x in the range of 0.1-0.67. At low AI content a significant A! surface enrichment suggests that amorphous AIOy on MgO surface blocks the basic sites. This enrichment decreases for higher AI content. As previously proposed, the migration of AI creates defects. The density and strength of basic sites increase. When segregation of spinel phase occurs, the density of basic sites decreases drastically. This evolution of the density of sites with the molar fraction in aluminium is confirmed by CO 2 adsorption (Fig. 1). Lamellar structures as catalysts

Altough the mixed oxide components were extensively studied, less attention was paid to the catalytic activity of the lamellar structures. Nevertheless Reichle (3) in his pioneer work found substantial conversion levels for acetone oligomerisation with catalysts activated at 450 ~ C and still retaining the lamellar morphology. More recently Constantino and Pinnavaia 2+ 3+ 2(24) reported that Mg -A1 -CO 3 LDH activated below 450 ~ could lead to highly basic selectivity in the conversion of 2-methyl-3-butyn-2-ol readily used as an acid-base reaction test (25). The most active catalyst, one order of magnitude higher, was obtained through thermal decomposition of LDH at 250 ~ C rather than by forming the mixed oxide at 450 ~ C. The former sample, in the lamellar form, was carbonated and totally flee of interstitial water. Similarly a Mg~0Al2 (OH)24CO3 sample activated at 110~ C acts as an efficient catalyst for the base catalysed epoxidation of olefins (26). As expected a dramatic influence of the interstitial anions was evidenced in the LDHs samples activated under these mild conditions. Indeed the carbonate and hydroxide intercalated LDHs were the most basic ones (6). The latter corresponds to the meixnerite-like structure. Several recent reports emphasized on the high efficiency of these meixnerite-like forms to catalyse reactions currently performed with NaOH or KOH in liquid phase (27-31) owing to the presence in these materials of OHgroups acting as Br6nsted basic sites. These sites are generated by rehydration, under decarbonated water, of thermally activated Mg/AI-LDH using their memory effect (1). 2+ The above results evidence the versatile character of the catalysts obtained from Mg .~j3+ 2-CO 3 LDHs, whose properties could be tailored through the chemical composition and the activation conditions to perform differently demanding basic reactions. A good illustration arises when one considers the aldol condensation of acetone (Fig. 2). In this reaction diacetone alcohol (DA) is the first product which then rapidly dehydrates to yield mesityl oxide (MO) and mesitylene (IMO) as a by-product. The aldol condensation of MO, previously deprotaned in r and an other acetone molecule may lead to phorone. Finally cyclization of phorone, involving an internal 1,6 Michael addition, leads to isophorone (IP) (32).

333

o

H=C' ~ I'I=C~ C H s

..cvc CH=

\

C=O

/ Cl-~

C--OH

+ A~.--t~. I ~

~ / o l cond.

Acetone

..c c., - H,O

ICH= ~

9

C-O

I

CH, DAA

1,0 Mlchael/ I

-o~a~on/

IP

C

U

Ib OH I

+ ~=ao~ Tdmedc "~;H,O ~ Intermediates _\+ ~-=o~

C=O a/dol cor I C~

TI

. =x<,~o

il

Tetramers

MO

=' . ~ H,c 0% Meaitylene

1

Higher polymers

Figure 2: General reaction scheme for the condensation of acetone With samples activated at 400 or 500~ (3,16), therefore totally dehydrated and decarbonated, and operating in vapor phase at 200~ and 100 kPa, the selectivity reaches practically 100 % into MO + IMO + IP. With the catalyst similarly activated, and operating at 0~ in liquid phase, DAA and MO are obtained. While using as catalyst the meixnerite-like form, the selectivity towards DAA is 100 % for a conversion reaching the thermodynamic equilibrium (23 %) (27,28). These results can be readily explained by the different nature of the active sites involved: acid-base pairs for the obtention of MO, IMO, IP and Br6nsted basic OH sites for the obtention of DAA with the meixnerite-like form. Selective hydrogenation of nitriles over base/metal bifunctional catalysts

Hydrogenation of nitriles is an important industrial reaction for the preparation of amines (33). A typical example is the hydrogenation of adiponitrile to 1,6-hexanediamine for the manufacture of nylon-6,6. Conventional catalysts consist in heterogeneous transition metals such as Raney Ni and Co or supported Ni. Due to the high reactivity of partially hydrogenated intermediates, the reaction leads to a mixture of primary, secondary and tertiary amines. Operation process needs the use of base promoters in the medium in order to poison the acid sites largely responsible for coupling reactions between the strongly basic reaction products (34,35). The gas phase hydrogenation of acetonitrile provides a typical example of the potential of LDH-like precursors for tuning the selectivity in such a hydrogenation reaction (36,37). A series of Ni/Mg/AI-LDH precursors was prepared (Table1) by co-precipitation of the appropriate mixture of nitrate salts under controlled pH. Catalysts were then activated by calcination at 350~ and reduction at 450~ and characterized by a combination of techniques, among them microcalorimetry, to determine the heats of adsorption of hydrogen, carbon monoxide, acetonitrile and monoethylamine (MEA).

334 Table 1 Composition, main characteristics and activity of Ni/Mg/Al-catalysts in the hydrogenation of aeetonitrile (Pro= 88 kPa., P,~to,~l~ 13 kPa). Sample #

Ni/Mg/Al (mol)

SBET

SNi

(m2/g)

(m2/g)

Temp.aso% (~

74/0/26 66/6/27 50/15/35 34/37/29

193 202 210 210

13 13 8 -

114 98 119 177

1 2 3 4

,

SeLbso% (mol.%) 90.4 91.2 93.8 75

8

Reaction temperature for 50% conversion, bSelectivity at 50~ conversion. The catalytic results were consistent with the bifunctional mechanism Q~mphmo Motml~function MigrsUon AcklOcfunclon proposed by Verhaak et al (35) and depicted in Figure 3, where acetonitrile undergoes hydrogenation to ethylimine and MEA at the Ni metal sites. Both compounds may then migrate to the acid sites where transamination reactions occur between protonated imine and MEA molecules leading to secondary amines. Although some contribution of the metal to 9 leh condensation reactions cannot be en.eNranro-oh H totally ruled out (38), the bifunctional mechanism has been supported by experiments performed on mechanical ; ; i ! : mixtures of a Ni/MgO catalyst with _~~m~o~. ~,~~c~~w,,[ either acidic, neutral and basic - ! . . aluminosilicates (35,36). The latter established moreover that the rate of the overall process is determined by the hydrogenation of the nitrile to the Figure 3 9bifunctional mechanism for the imine but the selectivity depends on the hydrogenation ofacetonitrile(35), competition between the subsequent hydrogenation of imine to primary amine on Ni sites and the transamination reaction which can occur on acid sites. Returning to the data of Table 1, though all catalysts exhibited fair selectivities towards MEA, it is clear that the amount of Mg in the LDH precursor influences both the activity and the selectivity. Regarding the latter, this statement becomes even more obvious when considering the selectivity into MEA at c a 5% total conversion (Fig. 4); ie under conditions where the surface has not been poisoned by intermediate basic products. The low activity of sample 4 was attributed to a lower reduction degree of Ni. Tichit et al (39)

(9

|

Y

41

~

. . . .

;

335 have reported TPR and XDR results showing that in takovite and hydrotalcite-like materials the reduction temperature of Ni remains roughly unchanged up to M g ~ g + Ni values of 0.4 and then significantly increases above this threshold. The lower hydrogenation activity of the catalyst favours moreover the occurrence of consecutive reactions of the intermediate imine, decreasing selectivity. In the case of the other three catalysts no significant differences in the properties (dispersion, heats of H2 and CO adsorption) of the metal sites nor in overall activity could be evidenced. The maximum in MEA selectivity obtained for a composition Mg/Mg + Ni of 0.23 (sample 3) is definitely attributable to a decrease of the acidity of the support, as demonstrated by the differential heats of adsorption of MEA (Figure 5), which are 40 kJ molq lower than on the sample free of Mg (sample 1). The changes in the acido-basic propertites of the Ni-catalysts after small addition of Mg can be compared to those obtained upon promotion of Ni/Al203 catalysts by potassium (35). Actually excellent performances leading to 92.6 % and 95.5% selectivity into primary amine at 99% conversion in the hydrogenation of acetonitrile and valeronitrile , respectively, have been reported for the Ni/Mg/AI systems described above.

100

", - = , i 9 9 ~ 9 / " " "

' 1 ""

"'

w' ' ' ' '

100

90

60

85

4o 8O

.~

20

75 70

j a j l l

0.0

0.1

0.2

0.3

0.4

0.5

0.6

O0

Mo/(Ma + Ni)

Figure 4: MEA selectivity as a function of M g ~ g + Ni molar ratio

I~0

....

I , , , . 1

200

300

....

I ....

400

500

Ethylamlne uptake (pmol g'~)

Figure 5: Differential heat of adsorption as a function of MEA uptake on sample 1,(A) 2 (0) and 3 (O).

Interestingly, the occurrence of a maximum in selectivity for the same reaction as a function of the Mg content was also reported for the Co/Mg/AI system (40). Such optima in Mg content are the result of a compromise between the reducibility of the metal and the acid character of the support, both decreasing with the incorporation of Mg. The balance between the two functions can also be altered through the activation conditions (41) and the use of bimetallic formulations of reducible cations (NiCo, NiCu). LDHs constitute without any doubt versatile precursors for bifunctional metal/basic catalysts with tunable properties. These materials are easy to prepare and, provided some minimal precautions are taken, easy to activate and handle. One can expect that in the next future they will find application opportunities, particularly for switching from discontinuous batch process operation towards fixed-bed continuous ones.

336 Mercaptan oxidation on solid bases catalysts

The catalytic oxidation of acid mercaptans and phenols into neutral disulfides or polysulfides is an ancient and mature process widely used in refineries for the sweetening of the white products. The reaction proceeds in the presence of an oxidation catalyst, frequently a metal phtalocyanine, associated with soda as co-catalyst. The mechanism of mercaptan oxidation can be summarized as follows: RSH + NaOH ---,RS" + Na + + 1-120 2M 2+ + 02 ---' 2M 3+ + 022. 2RS-+ 2M 3+ -*2M2+ + 2RS 2 R S - ' RSSR 022- + H20 -,2OH" +1/202 Soda is necessary in the process in order to produce the reactive mercaptide ion from the mercaptan in a first step. In order to overcome the problems associated with the use and handling of large amounts of caustic, considerable attention has been paid to replace it for solid bases. A recent contribution from the UOP Company (42) demonstrates the effectiveness of LDHs in this process which represents one of the first application of solid base catalysis to a commercial process. The catalysts have been prepared from Mg/AI (3/1) and Ni/Mg/AI LDHs precursors which were transformed into the corresponding high surface area (200 - 300 mZ/g) metal oxide solid solutions (MOSS) by calcination at 450~ for 6 - 12 h under flowing air. The MOSS were then impregnated with a methanolic solution of sulfonated cobalt phtalocyanine up to loadings in the 300 - 800 ppm range. The results of a pilot plant studies performed at 38~ using a CoPc-Mg/AI catalyst for the treatment of a FCC gasoline containing 149 ppm sulfur are shown in Figure 6.

LHSV

3

5

100

i"

g 96

Figure 6. Mercaptan oxidation of gasoline (42)

,~

94

t

~

IE

90~ .

o

.

.

.

.

so

.

.

.

.

.

.

.

ioo

.

~so

20O

Hours o n S t r e a m

Provided that a sufficient air supply was achieved (O2/RSH -- 0.5) over 99% of the mereaptans were converted into disulfides with no catalyst deactivation with time. Fast deactivation was in contrast observed when treating a kerosene feed. The three main causes for catalyst deactivation were found to be i) rehydration of the MOSS into the LDH form which resulted in a dramatic loss of surface area, ii) poisoning of the basic sites by the naphtenie acids present in the kerosene feed and iii) fouling by heavier hydrocarbon components in the kerosene.

337

I_HS% lOCI t! ~j _ ~

~)

~

4o

3

2O ..................... Feed R N I I

3 ,,,

i 323 ppm S i Ni-.MI~-A!

_

0

2O

40

Figure 7. Mercaptan oxidation of kerosene (42)

60

Hours

The addition of surfactant to the feed for preventing the build-up of heavy hydrocarbon species on the catalyst, the removal of the acidic poisons prior to the treatment and, above all, the addition of Ni to the LDH formulation, which is known to limit the rehydration of the MOSS into the parent layered structure, allowed to improve significantly catalyst stability as illustrated in Figure 7. In the example above, catalysts prepared by intercalating the phtalocyanine into the interlayer space of the LDH proved almost inactive owing to their low surface area. In addition, rehydration in the course of the reaction was deletorious and led to deactivation. Several examples exist however where intercalation of metal chelates has been advantageously used for the preparation of thiol oxydation catalysts. P6rez-Bemal et al (43) and Iliev et al (44), for instance, describe the intercalation of Co-phtalocyanine complexes into LDHs for the preparation of efficient catalysts for the oxidation of 1-decanethiol and 2mercaptoethanol, repoctively. The basic principle for the preparation is to allow the rehydration of the calcined mixed oxide back to the original LDH structure in an aqueous solution containing the tetrasulfonate or the tetracarbonate sodium salts of the complex under an inert atmosphere in order to prevent contamination by atmospheric carbon dioxide. XRD characterization of the catalysts revealed basal spacings of --- 2.3 nm. Since the van der Waals thickness of the brueite layers is about 0.48 nm, the observed spacings indicate a gallery height of--- 1.8 nm which corresponds to the van der Waals width of the carboxylated and sulfonated phtaloeyanine salts used. These data suggest therefore that the cobalt complex has been intercalated with the phtalocyanine ring perpendicular to the brucite sheets, as illustrated in Figure 8. J

, , - - , . w..

"o~

J

"-i

I

.-~l&

Figure 8: Schematic representation of the "edge-on" orientation of the (CoPcTs) 4 anion intercalated in a Mg/AI-LDH (41)

338 The properties of the intercalated tetrasulfonate Co-phtalocyanine (CoPcTs) were compared to those of the homogeneous catalyst for the autoxidation of 1-decanethiol at 35~ in the presence of a borate buffer at pH = 9.25 with an initial thiol/cobalt ratio of 154 (43). While the homogeneous catalyst had become totally inactive after a single catalytic cycle (150 turnovers) the supported one could achieve a total of 770 turnovers without detectable loss of activity. Moreover the basal spacing of the recovered catalyst remained unaltered at 2.3 nm. The fast deactivation of the homogeneous catalyst was in line with the specific behaviour of CoPc complexes (45,46) and results from aggregation and crystallization of the phtalocyanine molecules and formation of low active B-peroxo complexes. A similar effect has been noticed in thiol oxidation in the presence of Co complexes anchored on ion exchanged resins or other supports (46,47). Co-phtalocyanines intercalated into LDHs exhibit apparently enhanced stability through impeded mobility. Interestingly the relatively low surface area of the catalysts (> 30 m2/g) as determined by nitrogen adsorption indicates that the only cobalt centers available for reaction are located at external basal surfaces and crystallite edges. This means that the actual activity of the active centers is probably much higher than that measured from the overall composition. Improving the accessibility of the metal sites by optimizing the particle size and the separation between cobalt sites by controlling the Mg/AI ratio should allow significant gains in activity. A similar approach has been followed by other groups for the heterogeneisation of polyoxometaUate catalysts (48-50). Besides definite improvements in the stability of the catalysts, other advantages with respect to the homogeneous systems are highlighted, such as the possibility of using water as solvent for the oxidation of thiols by molecular oxygen over molybdenum-based catalysts (48), enhanced selectivities towards the oxide in the epoxidation of cyclohexene (49) and much higher specific activities in acetaldehyde oxidation (50) over heteropolyanions. These features suggest that not only the LDH structure plays a mechanical role in the immobilization of the active oxidation component but also it makes a direct or indirect contribution to the catalytic activity. In summary, the combination of a solid base with an oxidation catalyst undoubtly provides an interesting alternative for the preparation of friendly catalysts for the removal of environmentally undesirable contaminants of petroleum distillates and industrial effluents. Although many accounts demonstrate the validity of the strategy at the laboratory or eventually pilot plant scale, more has still to be done to improve the catalyst stability before anticipating practical application. By essence, the streams to be processed consist of complex mixtures that contain poisons or fouling agents which depress catalyst effectiveness. Efforts have therefore to be done in understanding the parallel and secondary reactions that lead to deactivation. Such a task is probably less gratifying than the quest for new catalytic formulations and reactions but proves necessary for filling the gap between highly promising models and real catalysts.

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