Jean-François Lambert

Adsorption and Polymerization of Amino Acids on Mineral Surfaces: A Review

The present paper offers a review of recent (post-1980) work on amino acid adsorption and thermal reactivity on oxide and sulfide minerals. This review is performed in the general frame of evaluating Bernal’s hypothesis of prebiotic polymerization in the adsorbed state, but written from a surface scientist’s point of view. After a general discussion of the thermodynamics of the problem and exactly what effects surfaces should have to make adsorbed-state polymerization a viable scenario, we examine some practical difficulties in experimental design and their bearing on the conclusions that can be drawn from extant works, including the relevance of the various available characterization techniques. We then present the state of the art concerning the mechanisms of the interactions of amino acids with mineral surfaces, including results from prebiotic chemistry-oriented studies, but also from several different fields of application, and discuss the likely consequences for adsorption selectivities. Finally, we briefly summarize the data concerning thermally activated amide bond formation of adsorbed amino acids without activating agents. The reality of the phenomenon is established beyond any doubt, but our understanding of its mechanism and therefore of its prebiotic potential is very fragmentary. The review concludes with a discussion of future work needed to fill the most conspicuous gaps in our knowledge of amino acids/mineral surfaces systems and their reactivity.

Silane layers on silicon surfaces: mechanism of interaction, stability, and influence on protein adsorption.

In this work the mechanism of (3-aminopropyl)triethoxysilane (APTES) interaction with silicon surfaces is investigated at the molecular level. We studied the influence of experimental parameters such as time, temperature, and concentration on the quality of the APTES layer in terms of chemical properties, morphology, and stability in aqueous medium. This was achieved using a highly sensitive IR mode recently developed, grazing angle attenuated total reflection (GA-ATR). This technique provides structural information on the formed APTES layer. The topography of this layer was investigated by atomic force microscopy in aqueous medium. The hydrophilicity was also studied using contact angle measurement. Combining these techniques enables discussion of the mechanism of silane grafting. Considerable differences were observed depending on the reaction temperature, room temperature or 90 °C. The data suggest the presence of two adsorption sites with different affinities on the oxidized silicon layer. This also allows the optimal parameters to be established to obtain an ordered and stable silane layer. The adsorption of proteins on the APTES layer was achieved and monitored using in situ quartz crystal microbalance with dissipation monitoring and ex situ GA-ATR analyses.

The molecular approach to supported catalysts synthesis: state of the art and future challenges

Abstract This contribution addresses some of the molecular-level aspects of classical supported metal catalysts preparation procedures. While the problems selected here have been chosen for their fundamental and practical relevance, important studies due to Knozinger and coworkers will be quoted in each case, and this short review may thus be regarded as a homage to their early activity in the field. We first briefly discuss the state of current knowledge on the surface species exposed at oxide surfaces, based on the particular case of γ-alumina. While considerable uncertainty remains as to the precise interpretations of spectroscopic data (especially in the presence of an aqueous phase), a concerted research effort might allow a molecular-level mapping of adsorption sites as a function of operating conditions. We then present the main mechanisms that may be operative in the establishment of metal-support interaction at the oxide–water interface, again with emphasis on the molecular interpretations. The distinction between strong and weak interactions is found not to be very helpful, while distinctions between selective and non-selective, or reversible/irreversible interactions, are more justified at the molecular level. Although our knowledge of these interactions is still lacunary, recent developments raise the hope of important progress in the near future. Molecular characterization during the later steps of catalyst preparation (thermal activation, calcination, reduction) is much less advanced. However, examples are presented in which the mechanism of initial metal-support interaction (established during the deposition step) has a demonstrated, lasting influence on all further steps of catalyst synthesis, suggesting the practical interest of these studies for fine-tuning of catalysts properties. Altogether, it appears that the molecular-level characterization of supported metal catalysts preparation is a realistic research program in the middle term.

Al-pillared saponites. Part 3.—Effect of parent clay layer charge on the intercalation–pillaring mechanism and structural properties

A series of synthetic saponites (smectite clays with tetrahedral substitution) have been intercalated by the Al13 polycation followed by pillaring (anchoring of the pillars to the layers on calcination to 500 °C).Even the clays with the highest layer charge have been successfully intercalated and pillared, but the amount of Al intercalated never exceeds one Al13 per six unit cells. This limit appears to be due to steric constraints at the interface between the intercalating solution and the delaminated clay. Furthermore, there is a competition between flocculation and Al13 intercalation: low-layer-charge saponites flocculate quickly and polycation intercalation proceeds only slowly thereafter. The ordering (followed by X-ray diffraction) and surface area of intercalated and pillared samples are well correlated with the amount of Al13 pillars in the interlayers. 29 Si NMR chemical shifts have been correlated with the layer charge: 29Si spectra undergo systematic changes for pillared samples owing to the anchoring of the pillars to the clay layers. 27 Al NMR and IR reveal that the Al13 pillars keep their basic structure on heating to 500 °C, although they lose some terminal water ligands, leaving five-coordinated Als that constitute Lewis acidic centres. Bronsted acidity is also present, owing to the remaining H2O and OH groups on the pillars, and possibly also to Si—OH—Al groups formed in the tetrahedral sheets.These observations allow us to present a schematic picture of the reactions involved in saponite clay pillaring.

Molecular-level studies of transition metal–support interactions during the first steps of catalysts preparation: platinum speciation in the hexachloroplatinate/alumina system

This paper summarizes recently published results on the mechanisms of chloroplatinate adsorption onto alumina, as revealed by the joint use of 195Pt NMR and other spectroscopic techniques. Molecular models of solution species and conceivable adsorbed species are presented. It is shown that the adsorption of hexachloroplatinate-derived species is initially electrostatic, but accompanied by weak specific bonding to the surface (probably H-bonding; these are the “specifically adsorbed” ions of triple-layer theory). This initial electrostatic adsorption can be followed by a slow, thermally activated grafting, i.e., by the formation of inner-sphere complexes with two surface (AlOH) groups. Upon drying at elevated temperatures, grafting becomes irreversible. However, modification of the alumina surface by chlorination prior to Pt deposition efficiently prevents grafting, opening the possibility of a chemical control of adsorbed Pt speciation.

Al-Pillared saponites. Part 1.—IR studies

Al-Pillared saponites have been prepared from natural Na-exchanged Ballarat saponite. Acidic properties of these saponites have been studied by IR spectroscopy using pyridine as a probe molecule. Al-Pillared saponites exhibit both Bronsted and Lewis acidity. Lewis sites are evidenced in the parent Na-exchanged clay as well as in the pillared clays. Bronsted acid sites are not evidenced in the parent Na-exchanged saponite but Al-pillared saponites exhibit Bronsted acidity as shown by a pyridinium band at 1549 cm–1. Bronsted acid sites have been related to OH groups at 3740–3720 cm–1 and 3597–3594 cm–1. The same wavenumbers are evidenced for ν(OH) stretchings in acidified saponite and in Al-pillared saponites. It is assumed that an acidic attack of Si—O—Al linkages which may provide Bronsted sites occurs on the clay sheets during the pillaring process.

Limitation of XPS for analysis of wood species containing high amounts of lipophilic extractives

Chemical composition of Norway spruce and pine, two softwood species, has been investigated by X-ray Photoelectron Spectroscopy (XPS). Contrary to results previously obtained with beech wood, which allow to obtain information on bulk chemical composition from surface composition analysis, XPS analysis appears to be unsuitable for the characterisation of Norway spruce and pine wood chemical composition. Indeed, chemical compositions calculated from XPS data differ strongly from those obtained from microanalyses which are in good agreement with theoretical composition described in the literature. XPS analysis of both the softwood surfaces indicated high carbon contents explained by migration of lipophilic extractives to the surface under the influence of the vacuum necessary for XPS analysis. Nonvolatile extractives contained in wood were extracted and deposited on glass plates and analysed. Survey and detailed C1s spectra indicated similar signals to those recorded on wood surfaces. This phenomenon was not observed when samples had been previously extracted before analysis. These results strongly evidenced that extractives present in both species are able to migrate through resin canals from the bulk of the sample to the surface when put into ultra high vacuum. XPS presents, therefore, some limits in the case of the analysis of softwood species containing extractives able to migrate to the surface during analysis. This behaviour, difficult to control, could lead to erroneous interpretations due to extractives enrichment of the surface under the effect of vacuum.

Influence of ageing on MoO3 formation in the preparation of alumina-supported Mo catalysts

Abstract Raman spectroscopy and XRD are used to study the preparation of alumina-supported Mo catalysts, with emphasis on the molecular level understanding of the influence of the ageing step (i.e. leaving the catalyst in contact with water after impregnation and before drying) on Mo speciation. For incipient wetness impregnation with heptamolybdate ((NH4)6Mo7O24), long oxide/water contact times (ageing) favor alumina dissolution and consequently increase the formation and crystallization of the resulting compound, (NH4)3[Al(OH)6Mo6O18] on Al2O3. Precipitation of this compound leads to the formation of MoO3 after calcination. On the contrary, minimizing the oxide/water contact time (and therefore alumina dissolution) by freeze-drying the system after impregnation inhibits the formation and crystallization of (NH4)3[Al(OH)6Mo6O18] and therefore drastically limits MoO3 formation after calcination.

A comparative study of the catalysis of peptide bond formation by oxide surfaces

It is well-known that amino acids deposited on some inorganic oxides undergo peptidic condensation. It is seldom realised however that a large diversity of behaviours can be observed in such systems. Here we use the apparently simple case of glycine-non-porous silica as a reference system, in which glycine (Gly) dimerisation to diketopiperazine (DKP) is easy to evidence, especially when using TG in combination with NMR. We then proceed to compare it with other AA deposited on the same support on the one hand, with Gly deposited on other mineral surfaces on the other hand. In a final section, we provide more detailed mechanistic information on the glycine condensation process on silica, including kinetic data and a (13)C solid-state NMR follow up of the species at various stages of thermal condensation. The best mechanism to rationalise these data involves a crucial step of isomerisation from zwitterion to neutral glycine, and the participation of several distinct types of surface sites probably consisting of silanol ensembles.

Characterization of the porous structure and cracking activity of Al-pillared saponites

A well-characterized saponite was intercalated with [Al13]polyoxo aluminium cations. Its surface area was studied by nitrogen and ethylene glycol adsorption: the relative validity of both techniques in the case of pillared clays was assessed and compared to the theoretically expected value. The structural stability was investigated both in high-temperature treatments and upon aging under air at room temperature. The catalytic properties for cracking of a heavy petroleum feedstock were compared with those of a commercial FCC catalyst and with results from the literature concerning similar systems. Pillared Al-saponites present several interesting properties: thermally stable porous structure and low coking. Their study seems promising in understanding the transformation of the pillars during calcination owing to the location of the aluminium only in the tetrahedral sheets.