Lewis and Bronsted acid sites

Raman spectroscopy has provided information on catalytically active transition metal oxide species (e. g. V, Nb, Cr, Mo, W, and Re) present on the surface of different oxide supports (e.g. alumina, titania, zirconia, niobia, and silica). The structures of the surface metal oxide species were reflected in the terminal M=0 and bridging M-O-M vibrations. The location of the surface metal oxide species on the oxide supports was determined by monitoring the specific surface hydroxyls of the support that were being titrated. The surface coverage of the metal oxide species on the oxide supports could be quantitatively obtained, because at monolayer coverage all the reactive surface hydroxyls were titrated and additional metal oxide resulted in the formation of crystalline metal oxide particles. The nature of surface Lewis and Bronsted acid sites in supported metal oxide catalysts has been determined by adsorbing probe mole-... 

This review will endeavor to outline some of the advantages of Raman Spectroscopy and so stimulate interest among workers in the field of surface chemistry to utilize Raman Spectroscopy in the study of surface phenomena. Up to the present time, most of the work has been directed to adsorption on oxide surfaces such as silicas and aluminas. An examination of the spectrum of a molecule adsorbed on such a surface may reveal information as to whether the molecule is physically or chemically adsorbed and whether the adsorption site is a Lewis acid site (an electron deficient site which can accept electrons from the adsorbate molecule) or a Bronsted acid site (a site which can donate a proton to an adsorbate molecule). A specific example of a surface having both Lewis and Bronsted acid sites is provided by silica-aluminas which are used as cracking catalysts. 

Fig. 4 Structures of Lewis and Bronsted acid sites in zeolites.

Two such well studied systems are pyridine chemisorbed on alumina (15) and pyridine chemisorbed on silica-alumina (16). It had been previously shown that alumina contains only sites which adsorb pyridine in a Lewis acid-base fashion whereas silica-alumina has both Lewis and Bronsted acid sites. These two different kinds of sites are distinguishable by the characteristic vibrational bands of pyridine adducts at these sites (see Table I). Photoacoustic and transmission results are compared in Table II. Note that the PA signal strength depends on factors such as sample particle size and volumes of solid sample and transducing... 

Two types of probe molecules have been used for the detection of Lewis and Bronsted acid sites. The first involves the adsorption of relatively strong basic molecules such as pyridine, ammonia, quinoline, and diazines. The second kind involves the adsorption of weak base molecules such as CO, NO, acetone, acetonitrile, and olefins. The pioneering works of Parry27 and Hughes and... 

The effects of post-synthesis alumination on purely siliceous MCM-41 material with A1(NC>3)3 on acidity have been studied by FTIR, NH3-TPD, and IPA decomposition reaction. The FTIR results of pyridine absorption show that both Lewis and Bronsted acid sites are increased by the post-modification. The amount of NH3 adsorbed on the alumina-modified MCM-41 samples increases with the loading of Al onto the surface of MCM-41. Due to the improved acidity, the alumina-modified MCM-41 materials show considerably higher catalytic activity for dehydration of isopropanol than purely siliceous MCM-41. In addition, XRD and N2 adsorption results show that all MCM-41 samples maintained their uniform hexagonal mesoporous structure well after they have been subjected to post-synthesis alumination with the loading of Al species on Si-MCM-41 varied from 0.1 wt. % up to 10 wt. % (calculated based on AI2O3). 

Substitution of aluminum for silicon in a silica lattice increases the concentration of both Lewis and Bronsted acidic sites over that of pure silica [35]. These sites, together with those created by other minor glass constituents, create the surface acidity of glass fibers that we observed in this work and has been noted by previous investigators [9-17], We consider this acidity to be the primary effect that we observed in this study. 

The question of the acidity of silica, alumina and silica-alumina surfaces has always been of great interest to catalytic scientists. Previously, transmision infrared spectroscopy, particularly of pyridine adsorption, has been used to distinguish the presence of Lewis and Bronsted acid sites on oxide surfaces (24). The frequency shift of the surface OH group during adsorption now... 

Infrared absorption can be used to estimate the relative amounts of Lewis and Bronsted acid sites on cracking catalysts. Bases complex with Lewis acid sites while proton transfer to the base occurs at Bronsted acid sites. Each has distinct, well-resolved infrared bands. For example, pyridine forms a complex with the Lewis acid site and produces an infrared absorption band at approximately 1450 cm-1. Pyridinium ions form at Bronsted sites and produce an absorption band at approximately 1540 cm-1. The relative intensities of these two bands can be used to estimate the relative amounts of Lewis vs. Bronsted acid sites. 

The acid strength of the catalyst was probed by CO adsorption and IR spectroscopy.23 Low-temperature CO adsorption on oxidized WZ indicates the presence of both Lewis and Bronsted acid sites.20,21 Lewis acidity is attributed to coordinatively unsaturated Zr4+ sites, present both on pure Zr02 and on WZ. The acid strengths of these centers are enhanced by the presence of polytungstate species.20... 

Chemical modification of the ALPOs is required to create a new class of catalysts. In the metalloaluminophosphates (MeALPOs), the framework contains metal (Me), aluminium and phosphorus. Thus, it becomes possible to produce a wide range of active catalysts with Lewis and Bronsted acid sites and redox properties by the partial replacement of Al3+ by Me2+ ions (e.g. Co, Cu, Mg, Zn, etc.) in an ALPO framework (Thomas, 1995 Martens etal, 1997). 

Figure 9.4 Regions in a typical curve of differential heats of adsorption versus adsorbed amount. All regions (a, b, c, d) can be observed for zeolite samples presenting both Lewis and Bronsted acid sites, as probed by ammonia adsorption. For oxides presenting only Lewis acid sites, the regions a, c and d are observed.

Ammonia (pfCi = 9.24, proton affinity in gas phase = 857.7kj moT ) and pyridine (pfti = 5.19, proton affinity in gas phase = 922.2kJ moT ) are the favored molecules for probing the overall solid acidity, since both Lewis and Bronsted acid sites retain these molecules. However the use of IR spectroscopy or XPS is necessary to distinguish qualitatively and unambiguously between these two types of sites. In addition to NH3 and pyridine, trimethylamine and triethylamine have also been used to probe the acidity of supported oxides. However, it has been mentioned [33] that these two molecules might not be able to equilibrate completely with the surface under typical experimental conditions. The use of substituted pyridines (2,6-dimethylpyridine) has also been considered in order to probe specifically the Bronsted sites [34]. 

Guler and Tunc (102) also studied the adsorption of chlorophylls a and b (mixture) on acid-activated bleaching clay using techniques similar to those they employed with p-carotene. They concluded that chlorophyll was initially converted to pheophytins, which were subsequently adsorbed on Lewis and Bronsted acid sites on the clay. 

Pyridine adsorption at room temperature on activated cloverite evidences the presence of Lewis and Bronsted acid sites. Pyridinium species hardly persists after evacuation at 423 K, showing that the Bronsted acidity is not very strong. Spectra analysis confirms that pyridinium species occur from the interaction with P-OH groups since the 3673 cm- and 944 cm- bands are regenerated by heating at the expense of the pyridinium species. The 3700 cm- v(OH) band hardly reappears by thermal evacuation suggesting that an irreversible reaction occurs during this treatment. Note that pyridine adsorption on phosphated alumina leads to protonation (8) due to interaction with free POH hydroxyls. 

The powder X-ray diffraction patterns were measured in a D-500 SIEMENS diffractometer with a graphite seeondary beam monochromator and CuKoj contribution was eliminated by the DIFFRAC/AT software to obtain a monochromatic CuKa,. The Unit Cell Size (UCS) was measured following the ASTM D-3942-90 procedure. The Surface areas were measured by nitrogen adsorption at 75 K on a Micromeritics Accusorb 2100 E equipment using the ASTM method D-3663-78. Temperature Programmed Desorption (TPD) of ammonia and pyridine adsorption by Infrared Spectroscopy (IR) were used to characterize the acidity of the zeolites. For IR-Pyridine the spectra were recorded each 100°C and the characteristic bands of Lewis and Bronsted acid sites (1444 cm" and 1540 cm, respectively) were integrated in order to obtain the total acid sites. 

The acidity distribution was obtained by pyridine adsorption-desorption (Table 3). The presence in the IR spectrum of a band at 1455 cm was assigned to the pyridine coordinatively bonded to Lewis acid sites, while a band at 1545 cm was attributed to the pyridinium ion using the known values of the extinction coefficient of the two IR signals [12] it was possible to determine the Lewis and Bronsted acid sites densities. The number of acid sites observed in the used catalyst was about half of that present in... 

Weng and Lee [85] found that the number of active sites, and of Lewis and Bronsted acid sites as well, increase with increasing amounts of niobium oxide in vanadia/titania catalysts. The highest activity of the promoted catalysts was found at 573 K. At lower and higher temperatures ammonia is oxidized. 

Infrared spectrum of pyridine adsorbed on undoped alumina-supported vanadium oxide catalyst, after evacuation at 150 °C, shows an absorption band at 1450 cm 1, characteristic for pyridine retained on Lewis acid sites, which has been related to V-free alumina [4,10, 11]. The intensities of the bands at 1450 cm l (related to Lewis acid sites) and 1545 cm I (related to Bronsted acid sites) have been used to determine the numer of Lewis and Bronsted acid sites on the surface of catalysts. The results are outlined in Table 1. 

To distinguish Bronsted from Lewis acid sites, FT-IR measurements of zeolite samples loaded with pyridine at 150°C were performed [11]. Intense bands at 1445 cm" and 1490 cm and only weak bands at 1545 cm were observed. Since the band at 1445 cm and 1545 cm can unambigously be assigned to Lewis and Bronsted acid sites, respectively, these findings are in agreement with the results from Al AAS. 

The mechanism of the reaction is unclear and probably both Lewis and Bronsted acidic sites play a role in the catalytic pathway. 

The activity of some Y zeolites showing different Lewis and Bronsted acid sites density is still studied in the acylation of dimethoxyarenes (with particular attention to veratrole. Scheme 4.7) with different acyl chlorides as a function of zeolite acidity and the lipophilic nature of acyl chlorides. Because of the particularly soft reaction conditions, namely, 65°C and 1 h reaction, 3,4-dimethoxyphenyl ketones 12 are the sole isomers recognized in the reaction mixture. Results of the catalytic tests (yield of compounds 12) confirm that the best catalyst is Y(14), characterized by an optimum ratio between Lewis and Bronsted acidity (medium-strength acid sites density for Lewis and Bronsted acid = 17.0 and 11.0 mmol x g- py. 

From the preceding experimental data and comments, it is clear that both Lewis and Bronsted acid sites exist on sulfated oxide catalysts, but the detailed role of fhe two types of acidity in various reactions, including Friedel-Crafts acylation, is still controversial. 

Infra-red spectroscopy in combination with pyridine adsorption as well as sodium ion exchange experiments were applied to CoAPO -5 and -11 (samples 9 and 6) in order to monitor changes in the number of acidic sites during oxidation and reduction of framework cobalt. Our results (Fig. 7) clearly show a considerable increase in the number of Lewis and Bronsted acidic sites and in the cation exchange capacity upon reduction of Co(III) to Co(II). In particular, the cation exchange capacity is almost quantitatively in line with the amount of framework Co(II) as determined by means of UV-VIS spectroscopy (Fig.6). 

Figure 7. Lewis and Bronsted acidic sites (determined by means of infra-red spectroscopy in combination with pyridine adsorption) and sodium ion exchange capacity of CoAP0 -5 and CoAPO -ll (samples 9 and 6) after calcination and after subsequent reduction by methanol treatment.

Ammonia, which possesses a large dipole moment, has been used extensively as a probe molecule for the characterisation of both Lewis and Bronsted acidic sites. Figure 22 shows the significant difference in the FR data between ammonia in zeohte crystals and in pellets. The FR spectra of ammonia in zeolite crystals demonstrated that the rate of the ammonia adsorption on different acidic sites in the crystals controls the overall dynamics of the processes occurring in the systems, hi the case of pellets, the rate-controlhng step was found to be macropore diffusion with (Fig. 22a,2,b,2) or without (Fig. 22c,2) surface resistances [77]. 

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