Structural Hydroxyls

Interaction of normal and iso-alkanes on protonated zeolites H-ZSM-5, H-mordenite and H-Y have been studied in detail by Eder et al. by a combination of calorimetry and IR spectroscopy (Table 7.5). 

At low coverages the hydrocarbons are adsorbed via H-bonds at the Bronsted acid sites. This is shown by loss of intensity of the sharp bridging hydroxyl resonance and the appearance of a broader band at lower 

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 ).  

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Many studies have been made of the rates of water evolution from layer-type silicate minerals which contain structural hydroxyl groups (clays and micas). Variations in composition of mineral specimens from different sources hinders comparison of the results of different workers. Furthermore, the small crystallite sizes and poor crystallinity that are features of clays limit and sometimes prevent the collection of ancillary observations (e.g. microscopic examination and diffraction measurements). 

The pH dependence of the adsorption of Co on montraorillonite in the pH range 5-6 has been interpreted (14,24) as due to the behaviour of structural hydroxyl groups on the clay or to hydroxy-Al compounds. In other work, adsorption of Cd, Co and Sr on to montmorillonite from solutions with widely different salt... 

Hydrated iron oxides can adsorb heavy metals. These adsorption properties arise from the presence of structural hydroxyl groups on their surface, which exhibit amphoterism (56) ... 

Frondel, C. (1982). Structural hydroxyl in chalcedony (Type B quartz). Amer. Min. 67 1248-1257. 

Fig. 2.19C. From Frondel, C. (1982). Structural hydroxyl in chalcedony (Type B quartz). Amer. Min. 67. Fig. 1, p. 19. With permission of Min. Soc. America and the author.

Russell, J.D. (1979) Infrared spectroscopy of fer-rihydrite evidence for the presence of structural hydroxyl groups. Clay Min. 14 109—114 Russell, J.D. Parfitt, R.L., Eraser, A.R. Farmer,... 

Thermal analysis, and to a lesser extent infrared spectroscopy, has shown that grinding kaolin reduces the number of structural hydroxyls and increases the amount of sorbed water. However, the infrared studies have been restricted to... 

HYDROGEN BONDS BETWEEN ADSORBED MOLECULES AND THE STRUCTURAL HYDROXYL GROUPS AT THE SURFACE OF SOLIDS... 

The number of four-coordinated A1 atoms is always greater than or equal to the number of structural hydroxyl groups (because two OH groups disappear per each framework A1 atom). 

However, the strongest arguments against the scheme of Uytterhoeven et al (203) come from NMR work, which is an excellent example of how H and 27A1 MAS NMR can be used in cooperation (168,170). The amounts of terminal and structural hydroxyls were separately measured using lH NMR, while the amount of four-coordinated Al was readily obtained from 27A1 MAS NMR. No three-coordinated Si or Al was observed the amounts of structural hydroxyls and four-coordinated Al were always equal and much less than half of four-coordinated Al was found after complete dehydroxylation. 

Uytterhoeven et al. (146) proposed that the protons liberated by decomposition of the ammonium ions attacked the oxygen atoms of the zeolite framework to form structural hydroxyl groups. The infrared absorption bands at 3650 and 3550 cm-1 were ascribed to hydroxyl groups of this type. The mechanism of formation of the hydroxyl groups is shown in the following two equations. 

Jacobs and Uytterhoeven (199, 200) observed a band in the 3700 to 3675 cm-1 region in addition to the bands reported by Ward. The intensities of the acidic bands at 3650 and 3550 cm-1 were greater than those observed by Ward, which probably resulted from a lesser degree of aluminum removal. The new bands at 3700 and 3600 cm-1 arose from hydroxyls that were nonacidic to ammonia (199, 200) and pyridine (198, 199), although bands from pyridinium ions were observed in the IR spectrum. The latter bands were attributed to interaction of pyridine with the 3650 cm-1 hydroxyls (200). Jacobs and Uytterhoeven (199) and Scherzer and Bass (198) attributed the 3700 and 3600 cm-1 bands to structural hydroxyl groups associated with removal of aluminum from the zeolite framework. The 3600 cm-1 band arose from weakly acidic hydroxyls (200) since the band was removed by treatment with 0.1 W NaOH solution. The 3700 cm-1 band was unaffected by a similar treatment. 

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). 

Pyridine and piperidine interact with the 3640 cm-1 hydroxyls (212), which resulted in their assignment to acidic structural hydroxyl groups similar to those found in the alkaline earth and hydrogen forms. The formation of the structural hydroxyls has been attributed by most investigators (219, 220) to the hydrolysis of the rare earth cations by adsorbed... 

The intensity of the 3640 cm-1 band is several times greater in rare earth Y zeolite than in the corresponding alkaline earth forms, which led Ward (2/2) to propose a further step in the hydrolysis mechanism, resulting in formation of two structural hydroxyl groups per cation ... 

In contrast with the observed behavior of the alkaline earth forms, addition of water to dehydroxylated rare earth ion-exchanged Y zeolite did not result in formation of new structural hydroxyl groups (2/7). This is consistent with Bolton s interpretation that a significant fraction of the structural hydroxyl groups are formed by exchange with protons in solution rather than by hydrolysis of the rare earth ion. 

Ward measured the o-xylene isomerization activities of Na, Mg, RE, and H—Y zeolites and found the rare earth form to be intermediate in activity between the magnesium and hydrogen forms as shown in Table IX (212). The sodium form was essentially inactive. He interpreted the activity relationship RE—Y > Mg—Y to result from the formation of two acidic structural hydroxyl groups per trivalent rare earth cation. The formation of acidic structure hydroxyl groups by exchange of sodium ions with protons in the rare earth solution, as proposed by Bolton (218), may also account for the greater activity of the rare earth-exchanged zeolite. 

Thermal activation of the clay up to 600°C results in a simultaneous loss of ammonia and structural hydroxyl groups. During decomposition, the ammonium ions release protons to the clay framework. Infrared spectra of dehydrated SMM samples exposed to ammonia vapor showed small amounts of ammonium ions and Lewis-bound ammonia. The ammonium ion was thought to result from interaction with silanol groups at crystal edges. Partially dehydrated samples adsorbed larger amounts of ammonia, and a greater proportion was present as ammonium ion, probably because of the lesser extent of dehydroxylation. 

Prior to sorption measurements, zeolite samples were activated by evacuation at elevated temperatures. There is frequently some question as to how precisely one can establish the mass of a zeolite sample from which all zeolitic water, but no water arising from collapse of structural hydroxyl groups, has been removed (l f ). In order to establish that the (zeolitic-water-free) masses of the activated zeolite samples used here are well defined, the following stepwise activation procedure was used. Each sample was first heated in vacuo at 300°C. When the pressure had dropped to below about 10 torr, the balance was isolated from the pumps, the rate of pressure increase measured, and evacuation resumed. This process was repeated until the rate of pressure increase fell to below 5 X 10 torr min l, a duration of time which was from 15 to 30 minutes. This is a rate such that were the increase due to water vapor alone, and were the rate to remain constant, the weight loss would still be undetectable after 2h hrs., a duration seldom exceeded in activating zeolites. 

The structure proposed by Bradley (1940) has three forms of water a zeolitic water, bound water (at the edges of the octahedral sheet), and structural hydroxyls. Using thermogravimetric curves, Caillere and Henin (1961a) attempted to measure the amount of these three types of water for several attapulgites (Table LV). The amount of bound water and hydroxyls differs considerably from that calculated on the basis of the ideal structure (column 5) and suggests there are more structural hydroxyls than proposed for the ideal structure. 

In order to study the influence of the decomposition of the TPA+ and TMA+ on the stability of the final material, a sample heated at 673 K and 1.33 10 2 Pa (in which TMA+ was still present) was exposed to moisture at room temperature for two hours. After this treatment, the IR spectrum shows the presence of a strong band at 1640 cm 1 characteristic of molecular water, which interacts with the hydroxyl groups of the SAPO (Fig. 2). The presence of species [H2O5] + (2500 cm 1) which have also been observed in HY zeolites should be noted (25). When the sample was dehydrated at 673 K in vacuum, the original spectrum was restored, indicating that the structure was maintained. However, if the same experiment is carried out at 873 K (Fig. 3), the subsequent exposure to moisture at room temperature results in the disappearance of the structural hydroxyl groups. The disappearance of these bands can be due... 

The high value for the quenching of 3,4-dimethoxyacetophenone by phenol suggests that it is probable that within the lignin structure hydroxyl groups are able to quench carbonyls by a static mechanism to yield phenoxy-ketyl radical pairs which decay on a timescales faster than the time resolution of our laser flash photolysis apparatus. Intersystem crossing rate constants for triplet radical pairs in the restricted environments of micelles have been demonstrated to be of the order of 2 -5 x 106 s-1 (25, 24). However, in the lignin matrix where diffusional processes are likely to be... 

The structural and spectral complexity of clay minerals is sufficient to consider a single mineral as a multicomponent mixture in itself. Detectible by near infrared spectroscopy are adsorbed water and structural hydroxyls (25.) exchangeable and structural transition metal cations (2fL and this work), adsorbed species including atmospheric gases (22), organic materials (2) accessory minerals (2SL) and, possibly, trapped hole centers (0 -centers). Thus it is of interest to adapt NIRA to studies of mineral surface activity. We have done this by examination of a small set of highly homologous clays in which laboratory control of only one variable at a time could be accurately achieved and independently confirmed. 

Figure 6. Absorbance spectra for selected clays in the region of water and structural hydroxyl absorption (1100-2500 nm). a) absorbance of SWy montmorillonite as a function of relative humidity, b) SWy, 100% Ca and Fe forms at a R.H. of 4%. Numbers in parentheses represent computer selected wavelengths giving maximal correlation coefficients in the region of interest.

Figure 7. Regression lines relating D2 of absorbance of crude SWy to measured water content at several key wavelengths characteristic of water or structural hydroxyl groups.

CORRELATION OF ABSORBANCE WITH WATER CONTENTS FOR VARIABLY EXCHANGED SWv CLAYS IN REGIONS OF STRUCTURAL HYDROXYL AND ADSORBED WATER... 

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