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Discurso del profesor Maria Ziółek

Con motivo de su investidura como Doctor Honoris Causa por la UNED


«Fascinating Niobium Catalysts»

Your Magnificence, Rector of the National University of Distance in Madrid, Members of the Academic Senate, Professors, research workers and students, Ladies and Gentlemen.

I am indeed much moved by receiving this honour and I wish to express my gratitude to the University authorities: His Magnificence Rector and members of the Academic Senate for supporting my candidacy for this highest distinction of Honoris Causa Doctorate. In particular, my heartfelt thanks go to the Promoter of this doctorate, Prof. Rosa María Martín Aranda, who dedicated much effort and with great passion, which is characteristic of Her, prepared and cared about further proceeding of this application. I address my thanks to the Head of School of Doctorate, Prof. Miguel Requena who has proposed my nomination. I am also much grateful to the reviewers of the application, Prof. Avelino Corma and Prof. Marco Daturi for their positive opinions. Today standing in front of you, Your Magnificence Rector and members of the Academic Senate who awarded me the honorary doctorate degree I must admit I have never expected this.

When I have started my research carrier, many, many years ago, I have never thought that my work will be so interesting, that I will meet so many clever and friendly people from all the world with whom I would collaborate in science, that I will work in so many laboratories in the world and teach so many fantastic students from the whole world and finally that I will be awarded with this highest and prestigious Honoris Causa Doctorate; awarded in UNED university, the place at which many of my students and co-workers had an opportunity to make research under great supervision of Prof. Rosa Martín Aranda. The place very close to my heart not only because of very good scientific collaboration with Prof. Aranda and her research team but also very friendly relationship with Rosa and her loved ones.

It is a great honour and pleasure for me to receive an Honoris Causa Doctorate from the UNED university during celebration of Saint Thomas Aquinas Day. It is great that you relate conferring of doctoral degrees to the greatest Doctor of the Church, St Thomas. Is it any relation between his philosophy and science? Sure, it is.

Thomas of Aquinas distinguished two different forms of knowledge. One is scientific knowledge gained by an act of inquiry permitting drawing certain conclusions, not previously known, from things we already know. The scientific knowledge based on an act of inquiry is contrasted with a kind of speculative activity that Thomas calls contemplation. According to Thomas an act of scientific inquiry aims at discovering a truth not already known, while an act of contemplation aims at enjoying a truth already known.

Maria Ziółek

The desire to learn unknown truths was at the basis of my interest in science, especially in the part of chemistry devoted to heterogeneous catalysis. I have asked many questions, why and how materials interact with substances and cause their transformations to other substances. These questions were the driving force of my search in heterogeneous catalysis.


Let me shortly explain what a heterogeneous catalysis is. This explanation is addressed to the non-specialist who are also present within this great audience. A catalyst, usually a solid in heterogeneous system, is a material which interacts with gas or liquid substances and makes them able to be transformed to other substances, called products (Figura 1).

The chemical reaction (this transformation) performed in the presence of the catalyst is faster than without it. It can be compared to driving a car from point A to B via pass or tunnel in the mountains and climbing through the mountains. Both ways allow one to reach point B, but the way through the mountains is more difficult, longer and sometimes impossible to overcome. Therefore we drive cars through tunnels or pass in the mountains because it is faster, with less energy consumption and consequently of lower costs. In heterogeneous catalysis solid is like pass or tunnel for a car on which substrates, in contact with the surface of solids, are transformed to products with less energy consumption than without the use of catalysts. Thus, catalytic processes are widely applied in many aspects of our life.

Most processes carried out in different industries such as chemical, petrochemical, refinery as well as in environmental protection require the use of catalysts. Moreover, catalytic processes occur in many spheres of our life. Let me mention the times of Roman Empire, when platinum cups were used for wine (Figura 2). If wine was kept for a longer time in such an opened pot, it went bad because alcohol in wine in the presence of metal (platinum) and oxygen from air was transformed to another compound (aldehyde) which not only tasted horrible but also was a poison. It is an example of the catalytic process in which platinum was a catalyst, alcohol was a substrate and aldehyde was a product of alcohol oxidation. Of course, in Roman Empire time the term ‘catalysis’ was unknown. The observations similar to that described above made over many centuries led to the formulation of the definition of catalysis by Berzelius on 1836.



Figura 1. Ilustration of catalytic process

Figura 1. Ilustration of catalytic process (C. Rothenberg, CATALYSIS - Concepts and Green Application, WILEY-VCH, 2008



Figura 2. Transformation of alcohol in platinum cup


Figura 2. Transformation of alcohol in platinum cup.

What is the idea of studying catalysis? I have mentioned the wide application of catalysts in industry which uses huge amounts of catalysts (measured in hundreds kilograms) put into the reactor through which the mixture of reagents flows or into batch reactor where the catalyst is contacted with liquid reagents. The interest is in effective, selective transformation of substrates to the desired products. What is the role of researchers in this area of industrial activity? Their role is to propose catalysts of proper composition and construction, which could be effective in a desired reaction and to optimize the conditions of catalytic process. To do this it is important to understand the mechanism of the catalytic reaction occurring with the participation of active centres on the catalyst surface, i.e. to perform investigation on the atomic level. The knowledge of the reaction pathway allows the optimization of the catalyst content and conditions of its working. Thus, the success in discovering of new catalysts or improvement of the catalysts already working requires, in general, deep knowledge in the fundamental and comprehensive chemistry as well as sophisticated methods used for characterization of catalysts. However, sometimes the work involves intuition, the speculative activity that St Thomas calls contemplation. It was the case with me when I started to be interested in a new element (new in the context of its use in catalysis), niobium, as a component of catalysts. It was in the 1990-ties. From that time, year after year I have become more and more fascinated by niobium containing catalysts.

Niobium is located in group five of the periodic table close to vanadium commonly used in catalysts. However, the properties of niobium compounds are totally different than those of vanadium ones and seemed not to be attractive for catalysis. Therefore, for a long time nobody was interested in niobium in the context of catalysis. In the 1970-ties Prof. Kozo Tanabe from Japan applied niobium(V) oxide as a catalyst in reactions requiring acidic active centres and showed that its activity was comparable with that of alumina, which was at that time commonly used in industry. Tanabe disclosed and demonstrated the unique properties of niobium(V) oxide as an acidic catalyst. However, this discovery did not cause a significant increase in scientific works devoted to niobium catalysts. Twenty years passed until the interest in niobium containing catalysts considerable increased, which is expressed by a significant growth in the number of papers devoted to niobium containing catalysts and the number of citations of these publications. There is no doubt that the increase in the interest in this field of catalysis was caused by the pioneering works indicating the possibility of incorporation of niobium into zeolites and ordered mesoporous silicas, which induced high activity in several catalytic processes, mainly oxidation and oxidative dehydrogenation.

The beginning of my interest in niobium catalysts dates back to the early 1990s. It was the time when Mobil Oil Company in the US discovered the possibility of synthesis of ordered mesoporous silica. Later on different structures containing free spaces (mesopores with sizes in the range between 2 nm and 50 nm) in the form of channels or cavities were synthesised (examples in Figura 3). The attractiveness of such materials comes from the possibility of their modification with different active species. The species can be well-dispersed in the silica-based supports which will still offer enough space in pores for including reagents and allowing their interaction with active centres.


Figura 3. Examples of mesoporous materials

Figura 3. Examples of mesoporous materials: SBA-15 (A). MCF (B) bases on DOI: 10.15199/62.2017.6.38 and MWW zeolites (C) [bases on DOI: 10.1016/j. cattod.201.


My idea was to incorporate niobium into mesoporous silica and my research team did it successfully for the first time in the 1990s [Figura 4]. It was a milestone in the development of niobium catalysts resulted in the significant increase in the number of papers devoted to niobium catalysts. The preparation procedure of niobium containing mesoporous silica is very simple. Selection of the synthesis conditions and precursors of elements used in the preparation procedure allowed the finding of the most effective ones. Many laboratories in the world started to use mesoporous niobiosilicates for different catalytic processes. It appeared that niobiosilicates, in which niobium species are isolated, exhibit totally different properties than niobium(V) oxide commonly used to that time.


Figura 4. Location of niobium in amorphous walls of hexagonally ordered mesoporous silica and active species formed after dehydroxylation

Figura 4. Examples of mesoporous materials: SBA-15 (A). MCF (B) bases on DOI: 10.15199/62.2017.6.38 and MWW zeolites (C) [bases on DOI: 10.1016/j. cattod.201.


What surface properties are generated after inclusion of niobium into silica structure? As shown in Figura 4, after dehydroxylation of niobiosilicate two very attractive catalytically species are formed: Lewis acid sites in the form of cationic niobium included into the silica lattice and Nb-O–radical species which reveal strong oxidative properties. Both kinds of active species are involved in the interaction with reagents and/or with the additional modifiers of the catalytic materials, like for instance noble metals. Thus, niobium in mesoporous silica creates acidity and redox centres.

In contrast, when niobium is included into zeolite crystalline structure, the basicity of the surface is enhanced. Zeolites are crystalline aluminosilicates in the classical form. They are originally microporous materials (micropores – the size up to 2 nm),although nowadays we have many possibilities to create mesopores in zeolites. Inclusion of niobium into the zeolite structure significantly changes the surface properties. Very spectacular is a great increase in the basicity of bridged Si-O- Al oxygen (oxygen # 3 in Figura 5).

The two examples shown above illustrate that niobium used as a component of different catalytic materials creates different active species and so a wide range of using niobium containing materials in catalysis has been proposed. Nowadays we have a huge number of papers showing different roles of niobium in catalysts: active phase, promoter, support component. Different materials containing niobium can be used in these roles. Let me specify the roles mentioned. The Active phase is the most important for catalytic process. Without the active phase the reaction rate is not enhanced. The active phase directly interacts with reagents and products of the reaction. The support is a material usually with high surface area to which active centres are anchored. Its main role is to provide good dispersion of the active phase and its stabilization. The Promoter is like a spice in kitchen which, when added to the dish in very small amount, increases its taste qualities. The promoter is an agent, which when added, often in small amounts, results in desirable activity, selectivity or stability effects.


Figura 5. Active species in niobium containing zeolites

Figura 5. Active species in niobium containing zeolites.

Different catalytic processes have been already successfully performed on niobium containing catalysts. A large variety of the catalytic reactions have been studied. Niobium containing catalysts have found different applications in such reactions as dehydration of alcohols towards the formation of valuable fuels, epoxidation of hydrocarbons, oxidation of alcohols to aldehydes and acids, removal of nitrogen oxides from exhaust gases from car engines, photocatalytic oxidation of dyes from waste water, and others. Let me give you examples illustrating each different role of niobium in catalysis.

Let’s start from niobium as the active phase. Niobium species are active in many oxidation processes. I will show you an example of the use of niobium containing catalysts in methanol oxidation with oxygen in the gas phase. Methanol oxidation can proceed via the oxidation route and the oxidation –acidic route. In the first step, methoxy species are formed at Brønsted acid sites (BAS) or Lewis acid sites (LAS). Next hydrogen is abstracted by nucleophilic oxygen from the catalyst and formaldehyde (FA) is formed. If FA is held at acidic sites of the catalyst surface strong enough it can interact with the next methanol molecule towards dimethoxy methane (DMM) or to be oxidized to formic acid (FA) and next to CO2. Formation of CO2 is an indicator of basicity. All the organic products which can be formed in methanol oxidation are commercial substances widely applied in many industries and everyday life. Among all products mentioned, formaldehyde belongs to the chemicals produced in the largest amounts (ca 10 Mt per year). Formaldehyde is applied as preservative, disinfectant, in wood industry and as one of the substrates applied in the syntheses of different organic compounds. Methyl formate is applied in cereal and tobacco crops as a fumigant, in the cellulose industry as a solvent, in foundries in the process of resin curing, in the synthesis of organic compounds, in the curing of phenol esters. Dimethoxymethane has found a great application in the perfumery as a propellant, in plastics industry, it serves as a reaction environment for conducting Grignard syntheses in food industry (extractant). Finally dimethyl ether is used as an extraction agent, in polymerization, as rocket fuel, starter for gasoline engines at low temperatures. So, you can see that from one substrate, methanol, one can obtain different products and the selectivity to one of them requires a special composition of the catalyst, and it is the task of researchers to propose this special composition and to perform the first laboratory tests.

Niobium active phase, depending on the surrounding, can be used as a catalyst component in methanol oxidation to a desired product. For the same activity, ca 40% of methanol conversion, dimethyl ether is the main reaction product over niobium (V) oxide, whereas niobium isolated in ordered mesoporous niobiosilicate catalyst is active in the oxidation of methanol to formaldehyde and methyl formate. Thus, it is clear that niobium in different surroundings leads to different products. Knowing the reaction pathway and the properties of different materials containing niobium one can construct a catalyst for the production of the indicated fine-chemical.

Another example of the use of niobium species as active phase could be the oxidation reactions in which hydrogen peroxide is used as oxidant. There are several such reactions, e.g. oxidation of glycerol to glycolic acid or oxidation of cyclohexene to epoxide, very important in the production of resins. A key feature of niobium species used in these reactions is its participation in the formation of extremely active superoxo species and hydroxide radicals from hydrogen peroxide.


Figura 6. Hybrid catalysts built from ordered mesoporuous niobiosilicates and organosilanes

Figura 6. Hybrid catalysts built from ordered mesoporuous niobiosilicates and organosilanes


The use of niobium containing materials as promoters can be illustrated by the fine chemicals and pharmaceutical production over hybrid catalysts, i.e. catalysts built of inorganic and organic components. If the inorganic moiety is niobiosilicate (ordered mesoporous material, e.g. SBA-15) playing also a role of the support, and the organic part is amino- or sulfoxo-organosilane, the obtained hybrid catalyst can be successfully applied in the Knoevenagel condensation or esterification reactions, respectively, leading to valuable products. The anchoring of organosilane on niobiosilicates enhances the basicity of amine groups and the acidity of sulfonic groups as a result of interaction between niobium and silane modifier (Figura 6). Sulfopropyltrimethoxysilane anchored on mesoporous niobiosilicate exhibiting the structure of cellular foams (MP-NbMCF) appeared to be very active and stable catalyst in the esterification of acetic acid with glycerol to triacetylglycerol (TAG), very important additive to biodiesel which improves the quality of this fuel. The stability of this catalyst is better than that of Nafion, often commercially used for this reaction.

Enhancement of basicity, very important in Knoevenagel condensation, by anchoring of aminopropyltrimethoxysilane onto NbMCF materials resulted in a very high activity in the production of the compound which is a very important intermediate in the production of medicines. Thus, niobium in the support used in the formation of this hybrid catalyst appeared to be an effective promoter for the active amine phase which made the higher yield of this important product.

One more example of spectacular action of niobium containing supports is its use for anchoring of gold. Gold is commonly known for its use in the production of beautiful jewellery or as a bank deposit. For many decades it was not considered as attractive element for catalysis. In the 1980-ties Prof. M. Haruta from Japan dispersed gold in the form of very small nanoparticles on the support and obtained extremely active catalysts able to activate oxidation of CO to CO2 even at temperatures below zero °C. From that time one of the main ideas in the construction of gold catalysts was the formation of gold nanoparticles as small as possible. One of the methods for obtaining this effect was the induction of very strong interaction between the support and gold particles which protects against agglomeration and permits stabilization of small gold nanoparticles (Figura 7). We have found that addition of niobium to the different supports allowed the stabilization of small gold nanoparticles. This discovery allowed us to produce several different catalysts effective in different catalytic oxidation processes. In this way we developed catalysts which can be successfully used in the production of glyceric acid from glycerol, formaldehyde from methanol, CO oxidation to CO2 and also photocatalytic degradation of dyes, important in the waste water purification.

Let’s say that our discovery of the attractiveness of niobium element in so many fields began from searching for something new, from looking for how to fill tabula rasa in this field not only in our mind but also in experimental experience. It is almost three decades of our interest in niobium catalysts after which I can conclude that niobium catalysts are fascinating and still can be developed. The expansion of such catalysts caused the discoveries of different possible applications in the production of many valuable compounds.

Figura 7. Gold and niobium containing catalysts

Figura 7. Gold and niobium containing catalysts

To summarize the most important features of niobium catalysts one can say that the possibility of very wide application of niobium containing catalysts stems from the unique properties of niobium species. They include easy formation of very active acid and redox centres which take part not only in the direct interaction with reagents, increasing the reaction rate, but these species are also involved in the anchoring and stabilization of different modifiers. Strong interaction of niobium in the support with different metals loaded on the support surface (e.g. gold) creates electron transfer between the components and leads to the collection of electrons on the surface of e.g. gold particles. Such a phenomenon leads to a very high activity, among others in total oxidation of wastes, possible because of very easy formation of superoxo species upon interaction with molecular oxygen used as reagent in many oxidation processes. Thus, my interest in niobium as a component of catalysts from three decades ago has become a small contribution to the local and international development of catalysis with niobium.

At the end of my talk let me come back to philosophy of science and research. The general principles in successful scientific research, not only in heterogeneous catalysis which is so close to my interest, can be defined, on the basis of my experience, in the following way (10 principles).

  1. One needs passion, diligence and stubbornness in the discovering of new or exploration of known phenomena and theories.
  2. Look for problems which are important from different points of view and have not been resolved yet or their solutions rise many doubts.
  3. Do not avoid difficult problems.
  4. Always seek the truth in explaining phenomena, their causes and effects.
  5. Define clear research hypotheses and methods which would allow you to confirm or reject these hypotheses.
  6. Decomposecomplex problem into severalconsecutive stages to explain.
  7. Look for proper analytical methods which can be applied for resolving the problem and apply them. If you have no access to such methods (equipment) look for institutions in the whole world which have them and ask for cooperation.
  8. Conduct a critical analysis of the results.
  9. Repeat the tests repeatedly if the results raise doubts. Test, improve, and test again.
  10. Share and discuss results of your research with local and international community.

Lord Rector of the National University of Distance Education, Dear Colleagues, Ladies and Gentlemen,

I hope to have shared with you the excitement and potential of the investigations of how to combine a deep insight into the atomic level of mater with the application of this matter in industry for development of methods used for the production of many valuable substances (e.g. medicines, cosmetics, polymers). Science and research have international dimension. Our cooperation with Prof. Rosa María Martín Aranda and her research team from National University of Distance Education in Madrid has been very fruitful, based on partnership and friendship and it has brought many important discoveries. I wish everybody such successful research collaboration.


Thank you


Madrid, 31 enero de 2019