Acid site concentration measurement

Figure 8.3 (Above) Heats of adsorption by calorimetry of pyridine on acid forms of ZSM-5 with different acid site contents, as measured by TPD/TGA measurements with isopropylamine, plotted as a function of loading (O, ISOpmolg , 360pmolg A, 600pmolg ). (Below) Heats of adsorption of pyridine on acid forms of different zeolites with different acid site concentrations, measured as a function of loading ( ZSM-12, 120 j,molg O Y, 160pmolg ZSM-5, 360molg A Mordenite, 800 J,molg ). [Data from references 20 and 21.]...

Further acid site strength and concentration measurements were reported by Morita et al. (164), who related the acidity measurements to various catalytic reactions. Using Y zeolite (Linde SK-40, 90% H form) activated at 450°C, they observed no acid sites stronger than an H0 of -8.2, although the total acid site concentration was almost twice that of the former investigations (Fig. 21, curve 4). They also measured acid site concentration as a function of decomposition temperature for NH4Y, and found that n-butylamine titration values paralleled results obtained from pyridine adsorption studies (41,151). The maximum total acidity occurred... 

Figure 21 illustrates some of the difficulties in comparing the results of different investigations. Although the four zeolite samples shown fall within a relatively narrow range of compositions, the measured total acid site concentrations differ by as much as 250%. In addition the acid sites of... 

Ai (140) measured the acid site concentration by the adsorption of ammonia. No correlation was found between the P/V ratio, the acidity, and the catalytic activity. This result has been attributed to the use of ammonia as a probe molecule that cannot distinguish between Lewis and Br0nsted acidity. Comaglia et al. (137) measured the acid sites using pyridine and acetonitrile as probes. However, the pyridine results showed no correlation between the Lewis to Br0nsted acid site ratio or the Lewis acid site concentration and the activity and selectivity of the catalyst for MA formation. 

Ai measured the acid site concentration by the adsorption of ammonia [85]. No correlation was found between the P/V ratio, the acidity and the catalytic activity. 

The chemical reagents used for the preparation of stock solutions were reagent grade and were used without further purifieation. The chemicals were in the form of the nitrate salts and were obtained eommercially from Aldrich or Fluka. Pure ceria samples were prepared by precipitation of ceria from aqueous solutions containing O.OIM Ce by adding 1 M NH3 solution. The precipitate was dried at 523K. The surface acid sites concentration was measured using Hammett titrations, with phenolphthalein as indicator, and a O.IM NaOH solution as base. 

A quantitative correlation between the chemical composition of the zeolite sample and the concentration of weak and strong acid sites (TPD measurements) has been established and summarized in the Table 1. 

Figure 4.13 Dependence of the concentration of Bronsted acid sites (as measured by the intensity of the band at 1540 cm r) on the catalyst preparation for supported boron trifluoride...

We also examined the adsorption of ethylamine and n-propylamine on the SAPO-5 samples. Again, the results were very similar to those obtained on H-ZSM-5 in that some of the amine desorbed as ammonia and the corresponding olefin between 625 and 700K [4]. Of particular interest was the fact that, for a particular sample, the number of moles which reacted for each amine was the same. This implies that each sample contains a discrete number of acid sites and that each amine samples the same sites. Using this as a measure of the acid site concentration, it is interesting to compare the site concentration to the concentration of substituted metals. This is shown in Table 2 for isopropylamine on the SAPO-5 samples and on MAPO-5(1) and CoAPO-5(1). For this comparison, we used the gel concentration as a measure of the framework metal ion content. 

It should be pointed out that the choice of probe molecule is very important in the measurement of acid site concentrations. In TPD-TGA ammonia and... 

The resultant materials were characterised by powder X-ray diffraction (XRD) and by nitrogen adsorption at 77K, using the adsorption isotherm to calculate BET surface area, and the desorption isotherm and BJH method to determine the pore size distribution. The concentrations of acid sites were measured by pH titration with standard NaOH solution, following exchange with excess NaCl solution. Water contents were measured by heating to constant weight at 150 C. 

Measurement of Acid Site Concentration and Strength in Microporous Solids... 

The concentration of Lewis acid sites was measured by the integral intensity of CO band in the range of 2170-2245 cm-i. For alumina and compositions with prevailing fraction of... 

The catalyst acidity is determined by the number of acid sites and their acid strength. The total concentration of acid sites, C<, can be obtained from independent TPD measurements. The average acid strength of the sites is characterized by the alkene standard protonation enthalpy,, and is typically determined by regression using reference... 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

For cationic zeolites Richardson (79) has demonstrated that the radical concentration is a function of the electron affinity of the exchangeable cation and the ionization potential of the hydrocarbon, provided the size of the molecule does not prevent entrance into the zeolite. In a study made on mixed cationic zeolites, such as MgCuY, Richardson used the ability of zeolites to form radicals as a measure of the polarizing effect of one metal cation upon another. He subsequently developed a theory for the catalytic activity of these materials based upon this polarizing ability of various cations. It should be pointed out that infrared and ESR evidence indicate that this same polarizing ability is effective in hydrolyzing water to form acidic sites in cationic zeolites (80, 81). 

DRIFT spectroscopy was used to determine Av0h shifts, induced by adsorption of N2 and hexane for zeolite H-ZSM-5 (ZSM-a and ZSM-b, Si/Al=15.5 and 26), H-mordenite (Mor-a and Mor-b, Si/AI— 6.8 and 10) and H-Y (Y-a and Y-b, Si/Al=2.5 and 10.4) samples. Catalysts were activated in 02 flow at 773 K in situ in the DRIFTS cell and contacted than with N2 at pressures up to 9 bar at 298 K or with 6.1% hexane/He mixture at 553 K, i.e., under reaction conditions. Catalytic activities of the solids were measured in a flow-through microreactor and kapp was obtained as slope of -ln(l-X0) vs. W/F plots. The concentration of Bronsted acid sites was determined by measuring the NH4+ ion-exchange capacity of the zeolite. The site specific apparent rate constant, TOFBapp, was obtained as the ratio of kapp and the concentration of Bronsted acid sites. 

A very convenient method to quantitatively determined the number of Bronsted add sites in the often used photochemical nano-vessels, zeolites X and Y, is available.28 This method take advantage of indicator/probe molecules which undergo an intense color change upon protonation within the zeolite pore network. The amount of a base necessary to quench the color change gives a direct measure of the concentration of acidic sites. The base used to titrate the Bronsted sites must be more basic than the probe molecule and sufficiently basic to be completely protonated. 

The lifetime of a zeolitic alkylation catalyst depends on the concentration of Brpnsted acid sites. This has been shown by Nivarthy et al. (78), who used a series of zeolites H-BEA with varied concentrations of back-exchanged sodium ions. The sodium decreased the concentration of Brpnsted acid centers, which led to a concomitant decrease in the measured catalyst lifetime during alkylation. 

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