IR pyridine adsorption

The acidity of the catalysts used here has been determined previously by IR pyridine adsorption experiments [5]. The amounts of acid sites (Bronsted/Lewis) at 300°C (mmol/g) are LaHY (0.133/0.089) and MSA (0.026/0.099). The total acidity found by IR is less than (< 1/20) the actual number of A1 present, not all A1 sites are accessible to pyridine. 

Bronsted and Lewis acidity of ZSM-5 zeolites loaded with transition metal ions (Co, Fe) via impregnation were investigated by Rhee et al. [682-684] using the IR/pyridine adsorption technique with thin sample wafers. 

IR pyridine adsorption studies show that the AL-PILC sample Vc calcined at 680°C (Vc-680) has a strong Lewis acidity only (Fig.4). On this sample steamed at 550°C (Fig.5) a strong Bronsted acidity is observed still detectable after evacuation at 480°C. Steaming at 650°C reduces the strength of both Brdnsted and Lewis acid sites,no more detectable after degassing at 300°C. 

IR spectra of the samples with and without pyridine were recorded on an FT-IR spectrometer (PE 430) with a resolution of 1 cm. In order to measure the pyridine adsorption, the samples were pressed into thin pellets, and placed into a quartz cell with CaF2 windows. The sample pellets were evacuated at 400°C for 2 h (< 10 Torr). After cooling down to room temperature, the pellets were exposed to pyridine environment (10 Torr) at 25°C. IR spectra were recorded after adsorption of pyridine for 1 h and evacuation at 150, 250, 350, and 450°C for 1 h. 

Figure 4.26 IR spectra showing absorbance bands due to pyridine adsorption on a H-FAU sample. PyrH is protonated pyridine on Bronsted sites, Pyr L is pyridine coordinated to Lewis acid sites, Pyr phys is physisorbed pyridine. All spectra recorded at 25 °C.

Figure 4.28 Effect of steaming and calcination on Bronsted and Lewis acid site strength distributions of a FAU-type zeolite as determined by pyridine adsorption/desorption IR.

It is important to understand the catalyst characteristics in detail, which in turn helps to understand the catalyst better and correlate the structure and composition of the catalysts with its performance, so that further improvement of the catalyst is possible. Acidity is an important property which influences the overall activity of the alkylation catalysts and the same was studied for Cui.xZnxFc204 by IR and TPD methods. The changes in acidity with respect to catalyst composition and temperature were studied through pyridine adsorption followed by IR measurements. In situ FTIR spectra of pyridine adsorbed on Cui xZnxFe204 between 100 and 400°C (Figme 23) indicated Lewis acidity is the predominant active centers available on the surface [14]. 

Figure 4. IR spectra of HYUS-8 sample before and after pyridine adsorption.

Infrared spectroscopic measurement was performed by Jasco FT-IR 230S using an in-situ cell. Hydrogen sulfide adsorption was carried out by introducing 40 Torr of hydrogen sulfide into the cell at 200°C, followed by evacuation at the same temperature for 0.5 hour. Pyridine adsorption was performed by introducing 10 Torr of pyridine vapour into the cell at 150°C, followed by evacuation at the same temperature for 0.5 hour. 

The Bronsted or proton acidity of the surface hydroxyl groups on silica and alumina is weak (51, 52, 53). Evidence for this comes from IR studies of pyridine adsorption, no surface pyridinium species (py H+)... 

The formation of structural hydroxyl groups in the presence of divalent cations has been explained on the basis of a hydrolysis mechanism (148) involving water initially coordinated to the metal ions (210, 214-216). The formation of a nonacidic hydroxyl group on the metal ion and an acidic hydroxyl on the zeolite framework by dissociation of the water molecule is consistent with the observed IR spectra and pyridine adsorption experiments. Further calcination at higher temperatures results in dehydroxylation and formation of Lewis acid sites at tricoordinate aluminum atoms in the zeolite framework (149). 

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

An additional difficulty in the determination of actual TOF values for zeolite catalysed reactions deals with the accessibility by reactant molecules to the narrow micropores in which most of the potential active sites are located. The didactic presentation in Khabtou et al.[37] of the characterization of the protonic sites of FAU zeolites by pyridine adsorption followed by IR spectroscopy shows that the concentration of protonic sites located in the hexagonal prisms (not accessible to organic molecules) and in the supercages (accessible) can be estimated by this method. Base probe molecules with different sizes can also be used for estimating the concentrations of protonic sites located within the different types of micropores, which are presented by many zeolites (e.g. large channels and side pockets of mordenite1381). The concentration of acid sites located on the external surface of the... 

Naturally, structures (d) and (f) do not exhaust all possible states of low-coordinated A1 atoms on the surface of the oxides considered. The calculations, however, seem quite sufficient to suggest that water molecule coordination by a LAS is energetically less favorable for aluminophosphate than for aluminosilicate surfaces. This conclusion is also in accordance with IR data, which indicate that LASs of the both oxides quite similarly interact with pyridine, whereas the LASs of aluminophosphates do not coordinate C02 molecules (136). Indeed, in the case of a sufficiently strong base (pyridine), adsorption interaction appears stronger than the structural coordination and therefore stabilizes the A1 atom in the adsorption state. On the contrary, for C02, which is certainly a very weak base, the interaction is strong enough in the case of aluminosilicates but is insufficient for the adsorption stabilization of aluminum in aluminophosphates. 

To elucidate the reason for the better performance of the acid-treated catalyst (B) we examined samples A and B with various analytical methods such as AAS, FT-IR, BET, pyridine adsorption, 29Si and27A1 solid NMR investigations. 

Dumesic and co-workers studied the activity of isopropanol dehydration (247) on a series of silica-supported oxide catalysts as well as the acidic properties of these materials using IR spectroscopy and TGA of adsorbed pyridine (59) and adsorption microcalorimetry of pyridine at 473 K (18,104). Samples that showed only Lewis acidity were at least one to two orders of magnitude less active than the samples that displayed Brpnsted acidity. The activity of the latter samples increased in the order Sc < Ga < Al + This is the same order found for differential heats of pyridine adsorption on the Brpnsted acid sites, and a good correlation between the heats and the activity was found. No correlation was found with the initial heats or for the samples that had only Lewis acidity. 

Acidity of the samples was measured by pyridine adsorption monitored by IR spectroscopy. Self-supported wafers were pressed and degassed in situ in the optical cell at 573 K. Then it was cooled to 473 K and pyridine was loaded. The wafer was kept in pyridine vapour for one hour followed by evacuation at the same temperature. Bands at 1450 cm and 1540 cm were used for the calculation of Lewis and Brpnsted acidity, respectively. 

Dealuminated M-Y zeolites (Si/Al = 4.22 M NH4, Li, Na, K, Cs) were prepared using the dealumination method developed by Skeels and Breck and the conventional ion exchange technique. These materials were characterised by infrared spectroscopy (IR) with and without pyridine adsorption, temperature-programmed desorption (t.p.d.) of ammonia. X-ray difiracto-metry (XRD) and differential thermoanalysis (DTA). They were used for encapsulation of Mo(CO)5. Subsequent decarbonylation and ammonia decomposition was monitored by mass spectrometry (MS) as a function of temperature. The oxidation numbers of entrapped molybdenum as well as the ability for ammonia decomposition were correlated to the overall acidity of the materials. It was found that the oxidation number decreased with the overall acidity (density and/or strength of Bronsted and Lewis acidity). Reduced acidity facilitated ammonia decomposition. 

XRD measurements at room and higher temperatures (up to 1275 K) were carried out with a Siemens D-5000 diffractometer, DTA analysis was performed using a Shimadzu DTA-50, whereas IR spectra were measured with a Perkin Elmer 325 spectrometer. Self-supporting wafers were activated under vacuum, then exposed to pyridine vapour at 475 K and subsequently outgassed at 475 K for 2 hours. Spectra were recorded before and after pyridine adsorption at the temperature obtained by the sample in the IR beam. 

The results of the acidity measurements for molybdenum-free samples via pyridine adsorption and IR spectroscopy are shown in Table 1 and Figure 1. 

IR spectroscopy was applied for studies of pyridine adsorption and desorption. These studies were carried out in a vacuum system. Zeolites were pressed into wafers ( 10mg/cm2) which were placed in a vacuum cell. Pyridine was adsorbed and desorbed at 475K. Higher temperatures of desorption were also applied. The spectra were recorded on Perkin Elmer spectrometers (Model 580 or 225). 

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