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O R I G I N A L P A P E R

Designing Heterogeneous Catalysts by IncorporatingEnzyme-Like Functionalities into MOFs

Karl Petter Lillerud Unni Olsbye

Mats Tilset

Published online: 8 May 2010 Springer Science+Business Media, LLC 2010

Abstract Everyone who works within the field of catal-

ysis draws inspiration from the amazing functionality ofnatures catalysts, the enzymes. It is particularly the mild

conditions that these catalysts are able to operate at and the

selectivity that they demonstrate that make these materials

dream targets for scientists involved in the art of synthe-

sizing homogeneous and heterogeneous industrial cata-

lysts. But enzymes also have their weak points; in

particular their low thermal stability and their often too

slow reaction rates for an economical industrial process are

problems that have to be overcome. The obvious solution

would be to copy the catalytic active center into a robust

open framework. A key property of an enzyme is its

selectivity; this property is partly regulated by steric con-

straints surrounding the catalytically active site. The

microporous zeolite based catalysts in some cases show

impressive selectivity based on the geometrical constraints

imposed by the size and shape of the regular channels in

these crystalline silicate and alumino-phosphate based

structures, and enzyme-like properties have been claimed

but the pure inorganic nature of the selective internal sur-

face in these materials makes it impossible to mimic many

important enzymatic properties. The new generation of

microporous materials, Metal Organic Frameworks

(MOFs) are hybrids of organic and inorganic structures.

This dualistic nature offers an unprecedented flexibility in

the possibility to incorporate both organic and metallicfunctional groups into the ordered crystalline lattice and

thereby opening up for a much greater possibility to copy

structural motifs known from enzymes into much simpler

but also more stable open structures. Several groups are

working on development of new catalysts by this approach.

Here we will illustrate this approach with structures that

mimic anhydrase and CH activation.

Keywords Metal Organic Frameworks

Catalytic selectivity Enzyme mimicking

1 Introduction

Nearly all new industrial processes in the chemical and

pharmaceutical industry are based on catalysis. In a society

where sustainability becomes increasingly important, a

chemists approach will be to develop more selective cat-

alysts that can minimize the use of raw materials and the

formation of byproducts. The most selective catalysts

known are natures own catalysts: enzymes. The dream of

many chemists, to be able to copy the functionality of

enzymes, is not new, but it is only in the last years that the

knowledge about enzyme structure and the mechanism in

enzyme functionality has reached the level where we can

see what the active site looks like and start to understand

how it works. But in order to make the dream come true,

man (chemist) must be able to make catalytic materials that

work in a similar manner as enzymes. We are unfortunately

still far from the goal of synthesizing structures in a

planned and controlled manner that are even close to the

complexity of the simplest enzymes, but we might be able

to copy some of natures ideas and implement this

K. P. Lillerud (&) U. Olsbye

Department of Chemistry, inGAP Center of Research-Based

Innovation, University of Oslo, Sem Selands vei 26, 0315 Oslo,

Norway

e-mail: k.p.lillerud@kjemi.uio.no

M. Tilset

Department of Chemistry, CTCC Centre for Theoretical and

Computational Chemistry, University of Oslo, Sem Selands vei

26, 0315 Oslo, Norway

1 3

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DOI 10.1007/s11244-010-9518-4

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functionality into manmade materials. A direct copy of the

enzyme is probably not the optimal catalyst for an indus-

trial process. Nature is forced to operate within a very

limited temperature and pressure range; temperatures must

be typically between 0 and 50 C (rarely up to 80 C) and

partial pressures much below one atmosphere [1]. By

contrast, typical industrial process conditions are temper-

atures [120 C and partial pressures[1 atm. The tem-perature has to be elevated to utilize the energy and an

increase in pressure will normally speed up the reaction.

Nature has elegant, selective processes but nature also is

too patientan industrial catalyst must have a higher

efficiency. In discussions with colleagues in the industry,

some provocative ideas have been suggested: It should be

possible to make materials that perform even better than

enzymes! [2].

One approach might be to copy the chemical environ-

ment around the active site and to build this structural theme

into structures that are considerable more robust than the

fragile enzymes. This approach will be discussed further.

2 Enzyme Structure

One important step in understanding how enzymes work as

catalysts is to solve the crystal structure with a resolutionthat

allows a detailed description of the atoms that form the active

site and the local environment around the active site in the

molecule. The first enzyme structure was solved by Blake

et al. and published in Nature in 1965 [3]. The resolution was

2 A and all details cannot be seen. The structure of the same

enzyme has now been redetermined at 0.65 A resolution [4],

a resolution that reveals essentially all necessary details.

Figure1 illustrates this development in protein (enzyme)

crystallography that to a great extent is a result of the

development of synchrotrons giving brighter X-ray sources,

the technique of freezing the crystals in liquid nitrogen and

not least the development of the computer power with

software needed to solve structures with this complexity. For

further reading on the development of protein structure

solution and structure presentation, the web sites: http://

www.umass.edu/microbio/rasmol/1st_xtls.htm and http://

www.umass.edu/microbio/rasmol/history.htm are highly

recommended.

But of course, making crystals of the pure protein of

sufficient quality is difficult; there are therefore still manyinteresting enzymes where the structure of the active site

has not been solved.

3 Enzyme Specificity, Active Site and Selectivity

The lock and key model that rationalizes enzyme spec-

ificity was put forward by Fischer already in 1894 [5]. It is

based on a purely geometrical viewpoint that rationalizes

some of the superb stereospecificity demonstrated by

enzyme catalysts, but it fails to explain the stabilization of

the transition state achieved by the electrostatic field insidethe cavity that embraces the reaction. Zeolites and related

oxide based microporous materials are manmade catalysts

that show a selectivity created by an analogous space

induced lock and key functionality [6,7]. In some cases

the selectivity is so specific that it is claimed to be enzyme

like [8]. Figure2 illustrates one of the complex organic

intermediates identified in the conversion of methanol to

olefins: this intermediate has a perfect geometrical fit to the

cavity, and transition states are also undoubtedly stabilized

by the electrostatic field inside the zeolite cavities. This and

similar selective reactions have, however, mostly been

discovered by pure serendipity. The nature of the internal

cavities in these inorganic materials is too different from

what we see in enzymes to allow a transfer of the knowl-

edge from enzymatic catalysis to these materials. Another

limitation with zeolite type materials is the problem of

synthesis by design. Although synthesis of zeotype mate-

rials has been one of our main research topics for the past

Fig. 1 Illustration of the

development in protein

(enzyme) crystallography. Left

Original illustration of the first

enzyme structure solved byBlake et al. in Nature, 1965 [3].

The resolution is 2 A and all

details cannot be seen. ( The

Nature publishing group, reprint

with permission). RightThe

same enzyme at 0.65 A

resolution [4] a resolution that

reveals all details

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http://www.umass.edu/microbio/rasmol/1st_xtls.htmhttp://www.umass.edu/microbio/rasmol/1st_xtls.htmhttp://www.umass.edu/microbio/rasmol/history.htmhttp://www.umass.edu/microbio/rasmol/history.htmhttp://www.umass.edu/microbio/rasmol/history.htmhttp://www.umass.edu/microbio/rasmol/history.htmhttp://www.umass.edu/microbio/rasmol/1st_xtls.htmhttp://www.umass.edu/microbio/rasmol/1st_xtls.htm

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25 years we must admit that it is a huge challenge to make

a new material with designed properties.

An emerging new class of microporous materials

Metal Organic Frameworks (MOFs), on the other hand,

has properties that are promising in this respect. A signif-icant portion of the internal surface of these materials is of

an organic nature and therefore more closely resembles the

surface that embraces the catalytic site in enzymes. So it is

possible to envisage that structural motifs from enzymes

might be recreated in these materials. Furthermore, as will

be discussed in more detail below, the possibility for cat-

alyst synthesis by design is much more feasible in MOF

materials when compared to the pure inorganic micropo-

rous materials.

Great structural and functional diversities are seen in the

active sites of enzymes, and we will choose as examples

enzymes where one or a few metals contribute to the cat-alytic center. One example is the zinc, cobalt or even

cadmium atom that constitutes the center of the active site

in one of the most studied enzyme systems, Anhydrase [9].

Figure3 is a simplified view of the zinc atom in a tetra-

hedral coordination environment, bonded to three nitrogen

based ligands and one water molecule. An organic hydro-

phobic cleft surrounds the site in this large enzyme mole-

cule. Figure4 illustrates the surrounding cavity and

compares it with a cavity of comparable dimensions in the

MOF structure UiO-67 [10]. Incorporation of heteroatoms

and functional groups to the linker in the MOF will further

increase the similarity to enzyme internal surfaces.

4 MOF Construction and Flexibility

The essential concept of MOFs, connecting coordination

complexes by the use of ligands that bind to more than one

complex and thereby forming extended structures, is not

new [11], but it was the appearance of the new and spec-

tacular structures like MOF-5 [12], HKUST-1 [13] and

MIL-101 [14] that really attracted new researchers atten-

tion toward this class of nano(micro)porous materials.

Figure5 illustrates the concept of extended MOF struc-

tures. A structural cornerstone might be formed either by a

single metal atom or, more commonly, by a metal cluster.

Another important advance in the development of the

MOF field was the concept of isoreticular series of struc-

tures [15] pointing to the possibility of a more systematic

design and synthesis of these materials. There has been

an animated debate about the possibility of doing synthesis

by design within the MOF community [11]. One reason for

this controversy might be the dualistic nature of the MOF

materials; inorganic cornerstones connected with organic

linkers. A reliable prediction of the synthesis and resulting

Fig. 2 Illustration of the complex organicinorganic complex iden-

tified as the active site in the conversion of methanol to olefins in H-

SAPO-34. The intermediate has a perfect geometrical fit to the cavity

of the catalyst SAPO-34. Transition states are undoubtedly stabilized

by the electrostatic field inside the zeolite cavity

Fig. 3 A simplified view of the active site in the anhydrase enzyme.

Zinc has a tetrahedral coordination with three nitrogen and one water

ligand. It is located in a well-defined organic hydrophobic cleft in the

large molecule

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structure of the inorganic part to a great extent still relieson serendipity. By contrast, the structure of the organic

linkers are normally preserved at the relatively mild con-

ditions for MOF synthesis and this part of the structure can

therefore be controlled to a large extent. This dual nature is

the reason for the enormous flexibility in these structures.

OKeefe et al. [16] have collected known coordination

complexes that are known to or might form cornerstones in

MOF structures. Figure6shows a small selection sorted by

coordination number. More complex inorganic structural

motifs like chains and sheets and even 3D inorganic

structures that are connected with organic ligands are also

known. Even for the structurally much simpler zeolites, thenumber of possible structures is in principle infinite. With

MOF structures is it possible to change the connection of

the inorganic cornerstone from two up to 12, and the

linkers might also have a variable connection, although in a

more limited range. This structural flexibility is illustrated

in Fig.7a, which addresses the variation in cornerstone

coordination, and in Fig.7b, which gives examples of

structures with the same cornerstone but different linker

connectivity.

5 Introducing Catalytic Sites in MOF Structures

Until recently, most of the focus has been on the synthesis

of new MOF structures, but this is now slowly shifting

towards applications. Examples dealing mostly with

adsorption and separation appear frequently, but other

applications and among them catalysis also attract

increasing attention [17]. There are in principle two ways

of introducing catalytic active sites. One approach is to

incorporate an uncoordinated site at one of the metal atoms

in the inorganic cornerstone. Figure8 illustrates this

Sodalite-type MOF structure with an expanded view of the

single Pd-atom cornerstone [18] at which Corma et al.

recently demonstrated catalytic activity [19]. An alterna-

tive approach that seems more appealing from the view-

point of mimicking enzymes is to build the active site into

the linker. Figure9 illustrates this principle that has been

suggested by several groups [20,21]. Figure10illustrates

ligands with catalytic centers.

Fig. 4 LeftThe cavity around

the active site in the anhydrase

enzyme. RightThe surface in

the octahedral cage of the UiO-

67 MOF structure. The internal

surface of MOF based

nanoporous materials have more

similarities to enzyme surfaces,

compared to oxide based

surface of zeolites and related

oxide based microporous

materials, see Fig.2

Fig. 5 Illustration of the MOF concept: a structural cornerstone

might be formed either by a single metal atom or, more commonly, by

a metal cluster which is connected by ligands that can form bonds to

more than one cornerstone. Bond energies are typically much larger

than what is found in coordination complexes Fig. 6 OKeeffe et al. have collected know coordination complexes

that are known to form or that might form cornerstones in MOF

structures. A small selection of these coordination complexes are

presented and sorted by coordination number, illustrating how natureprefers high symmetry structures. 131 different metal clusters are

known today (2009)

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Fig. 7 The flexibility in the construction of MOF structures are illustrated byaaddressing how variation in cornerstone coordination creates new

structures and b examples of structures with the same cornerstone but different linker connectivity

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asymmetric unit in this structure, with high temperature X-

ray diffraction data as an inset. The local platinum coor-

dination is as designed in the structure and the thermal

stability is very good, but with this system we were con-

fronted with a new general problem related to synthesis of

MOFs with active sites: This site will naturally coordinate

strongly both to the solvent used in the synthesis and to the

neighboring active site. The p-electrons on platinum tend

to coordinate to each other forming PtPt-stacking and

thereby blocking access to the active sites. Figure14illustrates this structure with nanoporous channels outlined

in blue. Guest molecules can access the structure but will

not reach the active sites.

A systematic search for alternative structures, through a

major part of the periodic table, was initiated both with

linker ligands that were uncoordinated to as well as pre-

coordinated to platinum. This resulted in a large number of

new and different MOF structures. Many of these exhibited

the familiar PtPt stacking, but several interesting new

structures without theses close contacts between the

potentially active sites were also found. One of the most

interesting new materials is a PtCu-MOF (Zeto et al., in

prep.). When working with this structure we realized that ithad several structural features that resemble the structure of

the methane monooxygenase enzyme [22]. The structure of

this enzyme has been solved at a resolution that reveals the

main features of the active site but still not all details, in

parallel with our synthetic work. Figure15 shows the

molecular surface around the active site and the

Fig. 13 The asymmetric unit in

the PtGd-MOF structure, with

high temperature X-ray

diffraction data as an inset

Fig. 14 The nanoporous channels in the PtGd-MOF structure

outlined in blue. Guest molecules can access the structure but will

not reach the active sites

Fig. 15 The electron density map and the interpretation of the

coordination around the metals that are believed to be the active part

of the enzyme, as presented in [22]. ( The American Chemical

Society, reprinted with permission)

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interpretation of the coordination around the metals that arebelieved to be the active part of the enzyme. Figure16is

an illustration of one cavity in our synthetic CuPt-MOF

this structure has motifs that are surprisingly similar to

parts of the enzyme structure.

The CuPt-MOF, which is structurally related to MOF-

505 [38] is sufficiently thermally stable to allow removal of

most of the solvent. The measured BET surface is 650 m2/

g, but the calculated surface is above 1000 m2/g. Probably

there are remaining DMF molecules that stay in the

structure, possibly at the Pt sites with consequential inhi-

bition of catalytic activity. Renewed attempts to remove

the residual DMF by supercritical CO2 extraction [39] isunderway. This example demonstrates that structures

resembling small parts of enzyme structures can be syn-

thesized, but we have to admit that this structure was found

by serendipity. However, in the example below we will

describe a successful planned synthesis.

5.2 Example ReactionAnhydrase

One of the most studied and also most important enzyme

systems is the carbonic anhydrase. This enzyme is vital for

all living organisms from primitive plants to mammals.

Nature has developed multiple modifications of this

enzyme, and in mammals alone 14 modifications are

known [9]. In addition to its crucial importance for all

living organisms that adsorb or expel CO2, the current

focus on climate change and CO2capture technologies has

created an industrial interest in anhydrase. Patents have

been filed that suggest large scale use of anhydrase in CO2capture [40].

Zinc is the active metal for the majority of anhydrase

enzymes, but there exist modifications of anhydrase with

cobalt and even cadmium [41] as the catalytically active

metal.

The reactive site in the enzyme is assumed to be a ZnOH

species (or other metals, i.e., Co, Cd). The proposed cata-

lytic cycle contains the steps of

reaction of ZnOH and CO2 to form a bicarbonate

complex; ejection of the bicarbonate ion and formation of a

Zn(H2O) complex;

proton dissociation to give a ZnOH moiety and a

protonated histidine group close to the Zn center.

Several attempts to mimic the active site of this enzyme

with coordination chemistry based on model Zn coordina-

tion complexes have been pursued, albeit with no success

in mimicking the active Zn site in carbonic anhydrase [42,

43]. The active site is proposed to contain a Zn atom in a

tetrahedral coordination sphere connected to histidine

ligands. Model studies [44, 45] have so far activated

CO2, but the process stopped at the bicarbonate complex(product inhibition).

Figure17 is an illustration of the Zn-complex from Ref.

[39] with water as the fourth, too strongly bound, ligand.

Thus, apparently the activation of water is an important

step in the proposed catalytic cycle. As a consequence the

pKa of the conjugate acid coordinated water molecule has

been the subject of some discussion [46]. In Zn containing

carbonic anhydrase the pKaof the water molecule has been

reported to be in the range of 6.97.7, whereas the pKarange quoted for [Zn(H2O)6]

2? is two units higher. This

has led to a proposal that the low pKa value for the coor-

dinated water molecule in carbonic anhydrase is in part aconsequence of the hydrophobic environment around the

active protein site. According to the authors of Ref. [39],

Fig. 16 A new CuPt-MOF from UiO showing structural units that

are surprisingly similar to the active site in the methane monooxy-

genase enzyme

Fig. 17 The Zn-complex from Ref. [39] with water as the fourth, too

strongly bound, ligand

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Structural studies show that the zinc atom in Zn(II) car-

bonic anhydrase is situated at the bottom of a cleft, lined

with hydrophobic groups in the protein molecule. It has

been postulated that this local hydrophobic environment is

largely responsible for the decreased pKa of the coordi-

nated water molecule.

It is therefore necessary to mimic both the metal site and

the local environment with the right hydrophobic character.This can probably be done by coordinating the active metal

inside a porous solid that has a hydrophobic internal sur-

face. Cobalt coordinated to a bipyridine linker in a modi-

fied UiO-67 may well be a suitable candidate. Figure18

illustrates such a structure with cobalt in this coordination

environment.

This MOF structure has also the property that it can

reversibly adsorb and desorb water at the Zr6-cornerstone.

This phenomenon utilizes the unusual property of zirco-

nium that it is stable both in a seven- and eight-coordina-

tion mode. This site, offering variable hydrophobic

character depending on the coordination, is located next tothe metal site like the histidine ligand in the enzyme.

6 Conclusion and Outlook

Approaching enzyme selectivity while maintaining indus-

trial type catalysts activity and robustness must be among

the ultimate goals for catalyst development in the twenty-

first century. The contributions to these proceedings from

Somorjai and Li address how advances in nanotechnology

enable the control of catalytically active structures on

surfaces. This contribution addresses the synthesis of

nanostructures inside nanopores. Enzyme specificity is a

result of the synergy between the active site itself and the

shaping of the environment around this site.

The overall structure of enzymes is far too complex to

be copied as manmade materials in the foreseeable future,but several groups, with some examples discussed above,

have demonstrated that small fragments of structural motifs

revealed by high resolution X-ray structure determination

of enzyme structures can be incorporated into MOF

structures. This allows recreation of catalytic sites as well

as their local environments. We have thereby moved a

significant step further in the direction of copying, not yet

the catalytic activity, but the idea or structural motives

from enzymes into manmade materials. These materials are

much simpler and more regular compared to enzymes, but

this is also a part of our goal since we aim at making more

stable materials compared to the enzymes.There is, however, much more to enzyme specificities

than their static structures. An important contribution to

both activity and selectivity of enzymes is their structural

flexibility and their dynamic behavior [47]. These are

properties that are much more challenging to transfer into

simpler, more regular, materials. Higher thermal stability

will inevitably also result in more rigid structures. A per-

fect transfer of the catalytic center with its local environ-

ment unaltered might therefore still not be sufficient to

recreate the catalytic properties.

Acknowledgments Elin Grahn, Orebro University is acknowledgedfor her help with enzyme structures. This work was supported by The

Research Council of Norway project no: 158552/431 and the Euro-

pean Commission (NMP4-CT-2006-033335 MOFCAT).

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