Adsorption on H-ZSM

Another thermal analysis method available for catalyst characterization is microcalorimetiy, which is based on the measurement of the heat generated or consumed when a gas adsorbs and reacts on the surface of a solid [66-68], This information can be used, for instance, to determine the relative stability among different phases of a solid [69], Microcalorimetiy is also applicable in the measurement of the strengths and distribution of acidic or basic sites as well as for the characterization of metal-based catalysts [66-68], For instance, Figure 1.10 presents microcalorimetry data for ammonia adsorption on H-ZSM-5 and H-mordenite zeolites [70], clearly illustrating the differences in both acid strength (indicated by the different initial adsorption heats) and total number of acidic sites (measured by the total ammonia uptake) between the two catalysts. 

Fig. 9 Isotherms of ethylbenzene adsorption on H-ZSM-5 for various adsorption temperatures amounts adsorbed (mmol ) as a function of the ethylbenzene partial pressure (Pa)...

Formation and reactivity of methoxy groups on CH3OH adsorption on H-ZSM-5 was the subject of a time-resolved in-situ FTIR investigation by Jentys... 

Formation of acetaldehyde via reaction of acetylene and water over Cd-exchanged phillipsite, mordenite, clinoptilolite, erionite, chabazite, and zeolites A, X and Y was investigated by the group of Kallo [907,908], while acetaldehyde adsorption on H-ZSM-5 was studied via FTIR by Diaz et al. [909], which indicated proton transfer with formation of crotonaldehyde and subsequent dehydration. At pressures higher than 400 Pa oligomerization occurred. 

Water adsorption isotherms were recorded in order to determine the impact of the alkaline treatments (i.e. changes in porosity) on water adsorptioa The adsorption of water within the pores of ZSM-5 is important to EtO filtratioa The surface of ZSM-5 is hydrophobic. Water adsorption on H-ZSM-5 occurs at silanol groups and Bronsted acid centers. Excessive water adsorption has the potential to reduce EtO filtration by firlly hydrating the Bronsted add site, thereby minirrrizing the adsorption arrd sirbsequerrt hydrolysis of EtO. Water adsorption isotherms are presented in Figirre 4. 

Micro calorimetry measurements have also been used to characterize the strength of the hydrogen bond of acetonitrile in H-ZSM-5 [71-73,75]. The differential heats of adsorption on H-ZSM-5 were approximately constant at 105 kJ mol up to a coverage of one molecule per framework Al, after which the heats dropped. For comparison, on silicalite the differential heats were also constant at ca. 60 kJ mol suggesting that the strength of the hydrogen bond in H-ZSM-5 is at least 45 kJ mol ... 

Figure 2. FTIR spectra of water vapor adsorption under increased pressures (a) 2.25 kPa, (b) 4.20 kPa and (c) 7.55 kPa at 100°C on H-ZSM-5 zeolite and spectrum of the activated zeolite at 450°C (d).

Figure 3 Variations with coverage of the differential heats of adsorption of ammonia on H-ZSM-5 (sample 1) measu-red at 150°C, (A), 200°C, (a), 250°C, (fe 300°C (0) and 400°C (+) The sample was outgassed at 400°C prior NH3 adsorp-tion. The meaning of the arrows is explained in the text.

Figure 4 Variations with coverage of the differential heats of NHg adsorption measured at 143°C on H-ZSM-11 samples outgassed at 400°C. Samples 3 (0) 4 ( ), 5 ( ) and 6 (A).

It is now well established that when a surface presents electron donor or electron acceptor sites, it is possible to ionize molecules of relatively high electron affinity (> 2 eV) or low ionization potential values, resulting in paramagnetic radical ions. For instance anthracene and perylene are easily positively ionized on alumina (7 ) (IP = 7.2 and 6.8 eV respectively). The adsorption at room temperature of benzenic solution of perylene, anthracene and napthalene on H-ZSM-5 and H-ZSM-11 samples heated up to 800°C prior to adsorption did not give rise to the formation of the corresponding radical cation. For samples outgassed at high... 

Figure 6a Variations with coverage of differential heats of NH adsorption at 143°C on H-ZSM-5 samples outgassed at 400°C not mentionned in table 1.

Figure 1.10 Differential heats of adsorption as a function of coverage for ammonia on H-ZSM-5 (o) and H-mordenite ( ) zeolites [70], In both cases, the heats decrease with the extent of NH3 uptake, indicating that the strengths of the individual acidic sites on each catalyst are not uniform. On the other hand, the H-ZSM-5 sample has a smaller total number of acidic sites. Also, the H-mordenite sample has a few very strong sites, as manifested by the high initial adsorption heat at low ammonia coverage. These data point to a significant difference in acidity between the two zeolites. That may account for their different catalytic performance. (Reproduced with permission from Elsevier.)...

Negishi, H., Sasaki, M., Iwaki, T, Hayes, K. F., and Yasunaga, T. (1984). Kinetic study of adsorption-desorption of methanol on H-ZSM-5 using a new gas-concentration jump technique. J. Phys. Chem. 88, 5564-5569. 

Figure 7.8 The relaxation of the aluminum tetrahedron upon adsorption of ammonia on H-ZSM-5. Adapted from ref [33].

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

Figure 22.4 IR spectrum of and H2 0 adsorbed on H-ZSM-5 (adapted from Fig. 2 in Ref [37]). Shown is the assignment based on frequency calculations for models of the neutral adsorption complex [34]. The overtone of the in-plane SiOH bending ( zqh) hilh ooto the red-shifted OH stretching frequency of the bridging hydroxy group of the zeolite, vOH. ...

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. 

The following IR bands being indicative of the adsorbates benzene, ethylbenzene, and p-xylene were monitored at 1478, 1496/1453, and 1516 cm , respectively. Sets of spectra of benzene or p-xylene on H-ZSM-5 analogous to that shown for ethylbenzene (Fig. 3) were monitored and, using the appropriate cahbration curves, the corresponding adsorption and desorption curves of the type displayed for ethylbenzene in Figs. 8a,b obtained (see also discussion of Fig. 29 below). 

Adsorption of acetylacetone on H-ZSM-5 was IR spectroscopically investigated by Alexander et al. [738] in combination with Al-MAS NMR and C-M AS NMR. They showed that acetylacetone adsorption led to an increase of the signal of octahedrally coordinated Al not through producing Al(acac)3,but by forming a loosely bound adduct of acetylacetone to extra-framework Al-spedes. Acetylacetone proved to be more strongly held on Bronsted acid sites than on extra-framework aluminum. 

Another instructive example of a reaction monitored in situ by transmission IR spectroscopy is the alkylation of the toluene ring by methanol reported by Lercher and coworkers [820,890,891 ]. This work was related to the investigation of adsorption and co-adsorption of toluene, methanol and ammonia on H-ZSM-5 and H, Na-ERI by the same group (vide supra and [127,652,814-821]). The authors used a continuously stirred tank reactor [892-895]. Some conclusions drawn by the authors from their results of combined IR spectroscopic and gas chromatographic experiments were reported as follows. 

Similar trends of increasing heat of adsorption with C-number are seen for the iso-alkanes. For H-ZSM-5 their adsorption is less favoured than for the -alkanes whereas they are more favoured in the large-pore solids. This results from steric hindrance in the ZSM-5. As a more marked consequence of the steric features, the packing of the iso-alkanes is much less efficient than of the w-alkanes in H-ZSM-5, so that the uptake of iso-butane is only around one half of that of -butane. This ratio is much closer to 1 in large-pore mordenite (0.85) and H-Y (1). Still larger steric effects were noticed for the adsorption of bulky alkylbenzenes on H-ZSM-5, where initial heats of adsorption of isopropyl- and -butyl-benzene are much lower (50 and lOkJmol, respectively) than those observed for ethyl- and -propylbenzene ca. 80kJmol ). ... 

Differential molar heats of sorption have been determined calorimetricaUy by Thamm et aL [178] for benzene and cyclopentane on H-ZSM-5 type zeo-htes with different aluminum contents. The results showed that the structural OH groups represent energetically preferred sorption sites for benzene and presiunably also for cyclopentane. The occurrence of inflection points in the sorption isotherms as well as of pronoimced maxima in the heat of sorption curves of benzene and cyclopentane on the Al-poor H-ZSM-5 samples indicated that the influence of sorbate-sorbate interactions on the state of the sorbed molecules rose with decreasing A1 content. Heats of adsorption of benzene and ethylbenzene were also measured by Nieflen et aL [ 179] on a H-ZSM-5 sample (Si/Al = 33.6) and were 64.5 and 86.5 5 kj mol , respectively. 

Microcalorimetric measurements of the differential heats of adsorption of NH3 at 423 K were performed by Witzel et al. [77] on H-ZSM-5 samples with various Si/Al ratios (22.5, 33.6, and 33.3) activated in high vacuum at high temperature (1075 K). The heat values reported were respectively 175, 168,... 

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