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Atomically-Precise Methods for Synthesis of Solid Catalysts
By Sophie Hermans, Thierry Visart de Bocarmé The Royal Society of Chemistry
Copyright © 2015 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-829-3
CHAPTER 1
Synthesis of Well-defined Solid Catalysts by Surface Organometallic Chemistry
FRÉDÉRIC LEFEBVRE
Université Lyon 1, CPE Lyon, CNRS, UMR C2P2, LCOMS, Båtiment CPE Curien, 43 Boulevard du 11 Novembre 1918, F-69616 Villeurbanne, France
Email: lefebvre@cpe.fr
1.1 Introduction
The knowledge in homogeneous catalysis is very high, due to the conceptual advance of molecular organometallic chemistry. Typically, reports in homogeneous catalysis provide not only information on the catalytic performances (activity, selectivity and life time), but also, in most cases, a detailed mechanistic understanding of the catalytic system. The actual elementary steps of the reaction, directly derived from the principles and the investigations of organometallic chemistry, are usually described. This knowledge allows a predictive approach of these systems, mainly based on the fact that it is possible to have only one well-defined catalytic species in the system. Unfortunately, from an industrial point of view, homogeneous catalysis suffers from many disadvantages and very often heterogeneous systems are preferred even if they are ill-defined and less active. The development of better catalysts in heterogeneous catalysis has always relied on empirical considerations since it is difficult to characterize the really active sites on the surfaces, as the so-called 'active sites' are usually in small number(s). At the present time, the number of accepted 'elementary steps' is still limited to a few examples mostly demonstrated by means of surface science, and the predictive approach, based on molecular concepts, is rare. The concept of surface organometallic chemistry has been developed as a possible answer to this problem. Its main objective is the creation on a support (which can be an oxide, a clay, a polymer etc.), of organometallic fragments which will be well-defined and uniform along the entire surface. These species will be characterized by all available physico-chemical methods in order to have a description of the coordination sphere around the metal as precise as possible, as for homogeneous complexes. This strategy, initially proposed by the group of J. M. Basset, has also been developed by other groups (see Table 1.1 for some examples) and has been the subject of numerous reviews and books. In most cases the support was silica (flame silica, porous silica or mesoporous silica) but there are some examples using other oxides such as alumina or magnesia (Table 1.1).
The grafted organometallic complexes can then be modified by using the classical rules of organometallic chemistry leading to species which are potentially active in catalysis. In these compounds the support can be a mono-, di- or tripodal ligand. Recently, a new dimension was added to this chemistry by performing, prior the reaction with the organometallic complex, a reaction replacing the grafting sites by other species such as N–H, Si–H, or phenol groups.
As a consequence, it is now possible to prepare, on a surface, many organometallic complexes where the electronic and steric effects can be tuned easily. This knowledge now allows researchers to have a relatively predictive approach where the starting point is not the organometallic complex but a catalytic reaction. First, a catalytic cycle is proposed on the basis of the classical rules of organometallic chemistry. Second, a grafted organometallic complex which is one species involved in the postulated catalytic cycle is prepared. Third, the catalytic reaction is performed. Depending on the results the ligands around the metal are modified or, if the reaction does not proceed by the proposed catalytic cycle, another mechanism has to be proposed and tested.
We will describe here this surface chemistry via some examples mostly from work done in our laboratory. Our purpose will be to give the rules which will govern the reactivity of the organometallic complexes with the surface and not to compile a complete list of what can be made. We will first describe the grafting sites on the surface as this point will govern the reactivity of organometallic compounds. We will then describe the reaction of these sites with organometallic complexes and how the resulting species can be transformed into active sites before giving some examples in catalysis.
1.2 Grafting Sites of the Support
Organometallic complexes can be deposited on many supports, such as metals, zeolites, oxides or carbons. Depending on the nature, density and homogeneity of the reactive sites on the surface of these materials, different behaviors will be observed, leading to sometimes completely different catalytic applications. As an example we will describe in more detail silica, as it will be extensively used in the following, as it is the simplest support (see Table 1.2 for a non-exhaustive list of grafting sites on other supports). Silica can be considered as the simplest support as it contains only SiO4 tetrahedra linked by [equivalent to]Si–O–Si[equivalent to] bridges. It can be found in various forms such as silica gel, flame silica or mesoporous silica (like MCM-41 or SBA-15). In all cases, at room temperature, the surface is covered by hydroxyl groups [equivalent to]Si–OH and siloxane bridges [equivalent to]Si–O–Si[equivalent to] in interaction with adsorbed water molecules. Upon heating under vacuum at ca. 150 [degrees]C all water molecules are desorbed and the infrared spectrum shows mainly, in the v(O–H) domain, a very broad band between 3700 and 3500 cm-1 attributed to [equivalent to]Si–OH groups linked via hydrogen bonds. Upon heating at higher temperature, condensation between two neighboring hydroxyl groups occurs, leading to the evolution of water molecules and formation of [equivalent to]Si–O–Si[equivalent to] bridges. As a consequence, the intensity of the broad infrared band decreases and a new sharp band, attributed to isolated silanol groups, appears at ca. 3750 cm-1. After heating at 500 [degrees]C only isolated silanols are present. Their amount can be determined by chemical methods (such as by their reactivity with CH3Li) or physical techniques (such as quantitative solid-state 1H MAS NMR). The two values can be different as the first method will only quantify the hydroxyl groups accessible to the reagents while the second one will give an estimation of the total number of hydroxyl groups. In the case of microporous solids the difference can be very important. In the case of flame silica, which is non-porous, the two methods lead to an OH density of ca. 1.4 OH nm-2. Upon heating under vacuum at 700 [degrees]C the OH density decreases to ca. 0.7 OH nm-2. For such low values, one can reasonably suppose that the hydroxyl groups are far away from each other and so well-defined grafted organometallic isolated species will be expected upon reaction with these hydroxyl groups. This is the key point of surface organometallic chemistry. However, this view is not fully realistic even if it is sufficient in most cases; sometimes a more precise description of the silica support is needed for explaining the experimental data. First of all, there are not only monohydroxyl [equivalent to]Si–OH groups on the surface but also dihydroxyl ones =Si(OH)2, as evidenced by solid-state 29Si MAS NMR. These =Si(OH)2 groups give infrared bands at the same position as the [equivalent to]Si–OH ones and so they cannot be distinguished by this method. On the samples heated at high temperature the 29Si MAS NMR spectra become broader, preventing the separation of the different Si(OSi)4-x(OH)x(x=0–2) groups and so the presence of some =Si(OH)2 species cannot be excluded even if their amount is probably very low. Studies by 1H double quanta MAS NMR are more informative. This method allows the observation of protons pairs which are separated by less than ca.5 Å. As a consequence, not only pairs of protons involved in hydrogen bonds are seen but also protons at a slightly higher distance. An intense signal is observed for pairs of isolated protons even on silica treated at 700 [degrees]C (for which the OH density is 0.7 OH nm-2), probing unambiguously that the repartition of the hydroxyl groups is not fully homogeneous. Additional experiments by triple quanta 1H MAS NMR or spin counting were performed. These experiments showed that some hydroxyl groups are present as nests of three silanols located on a cycle containing six silicon atoms as those observed on the (111) face of cristobalite. In that case, the distance between the hydrogen atoms will be ca. 3 Å. One, two or three cycles can be adjacent.
Finally, upon heating at a very high temperature (ca. 1000 [degrees]C), highly strained cycles containing two silicon atoms and two oxygen atoms are formed by condensation between two adjacent silanols. These cycles are highly reactive even if their amount is low (0.14 nm-2 on silica dehydroxylated at 1000 [degrees]C while the amount of residual hydroxyl groups is 0.4 nm-2).
Scheme 1.1 summarizes the variety of species which are present on dehydroxylated silica as deduced from these studies. Depending on the nature of the silica (non-porous flame silica, mesoporous silica, etc.) the relative amount of these species will be different, leading then in some cases to different reaction products.
As shown above, even if silica can be considered as the simplest support, there is not only one species on its surface. For other oxides such as alumina the situation is more complicated and more than five different types of hydroxyl groups can be observed, with all their combinations, without taking into account the Lewis acid sites. The most complex support is probably carbon, as its surface contains a lot of functional groups covering all the fields of organic chemistry. This complexity of the surface support will have a consequence on the number of species which will be obtained upon reaction with organometallic complexes as the strength of the bond between the metal and the surface will be more or less strong. It is for this reason that preliminary studies are always made on silica and mainly on silica dehydroxylated at relatively high temperature (500 or 700 [degrees]C).
1.3 Formation of Grafted Organometallic Complexes by Reaction with One Hydroxyl Group
As we have seen above, the active sites of the support are mainly (if carbon is excluded) hydroxyl groups; only their distribution and strength depend on the treatment and on the oxide under study. The formation of a chemical bond between the organometallic fragment and the solid will then pass, in most cases, through a reaction with these hydroxyl groups. We will describe here what will happen and by comparison of various supports and organometallic compounds how the reaction proceeds.
First of all, it is necessary to choose an organometallic compound with a M'–X bond for which the reaction M–OH + [M']–X[right arrow]M–O–[M'] + HX will be favored thermodynamically, M–OH being a hydroxyl group of the support. Many complexes can be chosen, such as chlorides or alkoxy derivatives. However, in these two cases the evolved hydrogen chloride or alcohols can further react with hydroxyl groups or M–O–M bridges of the support and so modify its properties. A typical example is the reaction at room temperature of tantalum methoxide Ta(OMe)5 with silica dehydroxylated at high temperature. 13C CP-MAS NMR of the resulting material shows clearly two signals for methoxy species. One of them can be attributed to a methoxy group on tantalum as expected and a second to a methoxy group on the silica support (such species can be synthesized by treatment at relatively high temperature of silica with methanol). These silica methoxy groups are formed by reaction of evolved methanol with a siloxane bridge or a silanol group. As this reaction does not proceed at room temperature in the absence of the tantalum complex, the metal plays the role of catalyst for this reaction. The consequence is that the grafting reaction will not be clean and the starting treatment of the support for the creation of isolated grafting sites will not be efficient as new potential grafting sites will be created during the reaction. It is for this reason that the organometallic complexes which will be chosen must lead to inert X–H species. The best choice is to have evolution of alkanes which cannot be activated by these supports. For this purpose the ligands around the metal will be alkyl, alkylidene or alkylidyne groups.
1.3.1 Reaction of Metal Alkyl Complexes
Very often homoleptic organometallic complexes are chosen as they will lead to only one surface complex, all ligands being equivalent. One problem is that, kinetically, the reaction will be slow, compared for example with that achieved with alkoxy compounds, due to the fact that the first step, the physisorption on the support, will not be favored, the interaction between hydroxyl groups and alkyl groups not being strong. To overcome this problem the complex can be sublimed on the support, avoiding the use of a solvent, but this method can be used only when it has a sufficient vapor pressure and sometimes the sublimation is accompanied by a partial decomposition. In all cases the observed reaction can be simply written as:
[equivalent to]Si–OH + [M]–R[right arrow][equivalent to]Si–O–[M] + R–H
When using homoleptic complexes and silica dehydroxylated at high temperature, well-defined species are obtained which are uniform over all the solid. This strategy has been applied to a lot of metals from the left (Ti, Zr, Hf) to the right (Sn, Ge) of the periodic table (see, for example, Basset et al.). In all cases the same result was obtained but the reaction did not proceed at the same rate: For metals of the left, such as Ti or Zr, the grafting reaction occurred easily at room temperature while for metals of the right, such as Sn, it occurred only at high temperature (ca. 180 [degrees]C). This is related to a different reaction mechanism as evidenced by the use of other supports with a stronger acidity such as cloverite, Y zeolite or heteropolyacids. Heteropolyacids such as H3PW12O40 are molecular compounds more acidic than sulfuric acid. Surprisingly, they do not react with alkyl complexes of Ti or Zr while they react at room temperature (and even below) with tin complexes. These results can be understood as follows. For metal complexes of the left of the periodic table, which are highly electron deficient with empty d orbitals, the grafting reaction occurs via an attack of the M–C bond by the oxygen atom of the hydroxyl group followed by evolution of the alkane. So the first step is the formation of the bond with the surface (Scheme 1.2). For metal complexes of the right, which have a high electronic density, the first step is an attack by the proton, leading first to the evolution of alkane and then to the formation of the metal–support bond. As increasing the acidity decreases the oxygen nucleophilicity this explains the different reactivity as a function of the acidity of the support. A consequence is that it is not possible to graft metal alkyl complexes of the left of the periodic table on highly acidic supports. Another consequence is that it is not possible to graft platinum methyl complexes on silica via breaking of the Pt–Me bond as these compounds are not stable thermally.
Another consequence of this mechanism is that it can be possible to graft metal complexes of the right of the periodic table by use of acidic species as catalysts: The grafting reaction of tetramethyl tin occurs at room temperature on H3PW12O40 supported on silica but the amount of evolved methane exceeds by at least one order of magnitude the number of protons of the polyacid and can only be explained by a migration on the surface. The mechanism (Scheme 1.3) passes through an attack of the tin complex by the acidic proton of the heteropolyacid followed by a migration of the grafted tin species on the surface and restoration of the acidic proton.
(Continues...)
Excerpted from Atomically-Precise Methods for Synthesis of Solid Catalysts by Sophie Hermans, Thierry Visart de Bocarmé. Copyright © 2015 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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