Surface Lewis Acid Sites Research Articles

Infrared study of the adsorption of octadecylamine at the MgO/CCl4 interface. A comparison with investigations of the adsorption of octadecylamine on SiO2 and Al2O3

The adsorption of octadecylamine on highly dispersed magnesium oxide immersed in carbon tetrachloride has been studied by infrared spectroscopy. No difference could be detected between infrared bands appearing in the spectra of MgO in situ immersed in the pure solvent after contact with the solution of octadecylamine and MgO drained and evacuated at beam temperature. Adsorption of octadecylamine on MgO involves coordinative interactions between amine and Lewis-acidic surface sites. The results are discussed in comparison with those for the adsorption of octadecylamine at the SiO2/CCl4 and Al2O3/CCl4 interfaces.

Infrared study of the adsorption of but-1-ene, buta-1,3-diene, furan and maleic anhydride on the surface of anhydrous vanadyl pyrophosphate

Bronsted-acidic hydroxy groups on the surface of vanadyl pyrophosphate act as catalytically active sites for the isomerisation of but-1-ene to but-2-enes. Progressive poisoning of the reaction is attributed to the formation of alkyl species, possibly alkoxy groups, on the (VO)2P2O7 surface. Adsorbed alkyl groups also result from buta-1,3-diene adsorption. Furan adsorbs on (VO)2P2O7 through coordinative interactions with Lewis-acidic surface sites. The presence of oxygen promotes the oxidation of furan to maleic anhydride.

Thermal desorption and infrared studies of primary aliphatic amines adsorbed on haematite (α-Fe2O3)

The adsorption of n-octadecylamine on α-Fe2O3 at the solid/liquid interface, and of n-butylamine at the solid/liquid and solid/vapour interfaces has been studied by infrared spectroscopy. To obtain further information about the nature of desorbing products, temperature-programmed desorption experiments were made with n-butylamine–α-Fe2O3 absorbates. No difference could be detected by i.r. spectroscopy between the nature of adsorbates formed under the various mentioned conditions. Adsorption of n-octadecylamine and n-butylamine on α-Fe2O3 mainly involves coordinative interactions between amine and Lewis-acidic surface sites (Fe3+ cations). Furthermore, hydrogen bonds are formed between surface hydroxy groups and adsorbed amine molecules. n-Butylamine adsorbed on α-Fe2O3 gave four different desorption peaks (I–IV) which are formed by n-butylamine (I: 423 K), butyronitrile (II: 530 K), CO2(III: 630 K) and H2O (IV: 713 K). Desorption of CO2 and H2O is caused by the oxidation of amine molecules strongly adsorbed on two different types of coordination sites.

Thermal desorption and infrared studies of butylamine adsorbed on SiO2, Al2O3 and CaO

The adsorption of n-butylamine on SiO2, γ-Al2O3 and CaO at the solid/vapour interface has been studied by infrared spectroscopy and temperature-programmed desorption(t.p.d.) The infrared spectroscopic results show that adsorption of n-butylamine on SiO2 leads to the formation of hydrogen bonds between surface hydroxy groups and amine molecules. Adsorption on γ-Al2O3 and CaO involves coordinative interactions between amine molecules and Lewis-acidic surface sites (Al3+ and Ca2+ cations), but hydrogen bonding also occurs. In the t.p.d. spectra one (SiO2) or two (Al2O3, CaO) strong maxima appear caused by the desorption of n-butylamine and butyronitrile, respectively. Desorption of unchanged butylamine at low temperatures can be attributed to amine molecules hydrogen-bonded to surface hydroxy groups of the oxides. However, there are some phenomena which indicate the possibility that molecules coordinatively bonded to weak Lewis-acidic surface centres contribute to this maximum. On the other hand, only butylamine molecules coordinatively bonded to strong Lewis-acidic centres are dehydrogenated and desorb as butyronitrile at higher temperatures.

Infrared study of water and pyridine adsorption on the surface of anhydrous vanadyl pyrophosphate

Infrared spectra of vanadyl pyrophosphate contained a broad maximum at 2550–3550 cm–1 which exchange experiments with deuterium oxide showed could be primarily attributed to residual OH-containing species in the bulk crystal lattice. However, the presence of Bronsted-acidic surface hydroxy groups was exhibited by the generation of adsorbed pyridinium ions when (VO)2P2O7 was exposed to pyridine vapour. Pyridine was also adsorbed on Lewis-acidic surface sites, some of which were converted to Bronsted-acidic sites by the adsorption of water.

Infrared study of the adsorption of propene and di-isopropyl ether on rutile

Infrared spectra are reported of propene and di-isopropyl ether adsorbed on rutile which had been preheated at 400 K. Propene was involved in hydrogen-bonding interactions with surface hydroxyl groups at 306 K and was probably also adsorbed through π-complex formation at Lewis-acidic surface sites. Chemisorption to give adsorbed isopropoxide ions occurred at higher temperatures and preceded subsequent oxidation to surface carboxylate species. Interactions between rutile and di-isopropyl ether were similarly dependent on temperature and involved hydrogen-bond formation, adsorption on Lewis-acidic sites and the appearance of infrared bands due to chemisorbed isopropoxide and carboxylate ions.

Infrared study of the adsorption of propan-2-ol on rutile at the solid/vapour and solid/heptane interfaces

Infrared spectra of propan-2-ol adsorbed on rutile have shown that non-dissociatively adsorbed propan-2-ol is coordinatively liganded to Lewis-acidic surface sites. Dissociative chemisorption occurs at exposed Ti4+ cations, particularly for dehydroxylated rutile, and generates isopropoxide anions. Surface hydroxyl groups on rutile were replaced by isopropoxide anions with the concomitant formation of water. Hydroxyl groups at sub-surface lattice sites were unaffected by propanol adsorption. Multilayer adsorption of propan-2-ol at high surface coverages involved the formation of molecular aggregates which were weakly bound by hydrogen-bonding interactions and which contained water molecules resulting from the chemisorptive reactions. Heat treatment of adsorbed propan-2-ol resulted in the formation of adsorbed carboxylate and possibly carbonate species. The implications of the results with respect to the dehydration of propan-2-ol catalysed by rutile are briefly discussed.

Infrared study of the adsorption of triethylamine at the rutile/vapour and rutile/heptane interfaces

The non-dissociative adsorption of triethylamine on rutile involves the formation of hydrogen bonds between surface hydroxyl groups and triethylamine molecules and coordinative interactions between triethylamine and Lewis-acidic surface sites. There is no evidence for the Bronsted acidity of hydroxyl groups.Dissociative chemisorption of triethylamine involves surface hydroxyl groups and generates adsorbed diethylamine, ethoxide ions and acetate ions as products. One type of hydroxyl group is completely unreactive towards triethylamine.

Measurement of surface acid site concentration by carbon-13 NMR

A method of measuring simultaneously the concentration of surface proton and Lewis acid sites is proposed. The method is based on 13C NMR spectroscopy of a suitable adsorbed amine. An investigation of several alkyl-substituted pyridines shows 4-ethylpyridine to be the most satisfactory choice. The method is tested on a silica-alumina cracking catalyst, and found to give good results for total acid concentration. Decomposition of the total acid into proton and Lewis acids requires a higher precision of measurement than presently achieved, but appears possible with existing instruments. Application of the method to silica-alumina with added water shows that the catalyst behavior in the presence of water is more complicated than assumed by previous workers.

Infrared studies of pyridine, 2,6-dimethylpyridine, and 2,6-di- tert-butylpyridine on stoichiometric and nonstoichiometric boron phosphate

The adsorption of pyridine, 2,6-dimethylpyridine (26DMPy), and 2,6-di- tert-butylpyridine (26DTBPy) has been studied on stoichiometric and nonstoichiometric boron phosphate by infrared spectroscopy. It has been shown that boron phosphate with P > B has a high percentage of Brønsted acid sites, while that with P < B contains predominately Lewis acid surface sites. Both 26DMPy and 26DTBPy interact with Brønsted and Lewis acid sites, and both can be displaced, the latter more easily, by pyridine.

Infrared studies of the adsorption of probe molecules onto the surface of goethite

Infrared spectra have been recorded of haematite in contact with ammonia, pyridine, nitric oxide, carbon dioxide, acetic acid and trimethylchlorosilane. Ammonia and pyridine were adsorbed onto weak Lewis acidic surface sites on haematite which had been heated in oxygen at 713 K. No detectable Lewis acidic sites existed on haematite which had been heated at 973 K. No evidence was observed for the adsorption of nitric oxide on haematite and carbon dioxide only gave a single weak band in spectra at 1320 cm–1. Acetic acid was chemisorbed to give surface acetato species possibly in three distinct configurations. Surface hydroxyl groups on haematite reacted with trimethylchlorosilane vapour to give hydrogen chloride, which was in part adsorbed, and surface trimethylsiloxyl groups. The relative reactivities of different types of surface hydroxyl group could be distinguished.

AN INFRARED STUDY OF THE ADSORPTION OF AMMONIA ON POROUS VYCOR GLASS

The infrared spectrum of ammonia adsorbed on porous glass at 20 °C and 150 °C has been studied in the region 1450–4000 cm−1. No absorption band due to the asymmetric bending mode of ammonia was observed but in the NH stretching region, bands occurred at 3280 cm−1, 3320 cm−1, 3365 cm−1, and 3400 cm−1. The bands at 3320 cm−1 and 3400 cm−1 were easily removed by evacuation and are due to ammonia molecules hydrogen bonded through the nitrogen atom to surface hydroxyl groups. The bands at 3280 cm−1 and 3365 cm−1 were not removed by evacuation even at 150 °C and are due to ammonia molecules held to surface Lewis acid sites by the nitrogen lone-pair electrons. The site for this adsorption is not a surface hydroxyl group. These results are further evidence for the existence of the two adsorption sites proposed by Folman and Yates. Deuteration of the surface OH groups was easily accomplished with D2O vapor at 300 °C and the rate of hydrogen exchange between adsorbed ammonia molecules and surface OD groups was found to be rapid.