A.M. Khalil

Heats of immersion of pure and doped magnesia in cyclohexane. Effects of micropores

Heats of immersion of pure and doped magnesia in cyclohexane were measured at 35°C for samples heated at different temperatures in vacuo. For the pure samples, rise in temperature of thermal treatment leads to a gradual decrease in the heats of immersion normalized per unit area (h1, ergs cm–2), till a certain limiting value of 44.5 ergs cm–2 is reached. Pore structure analyses lead to certain correlations between the heat of immersion and the fraction of the total surface located in micropores. Pores which could effectively play a role in altering the heat of immersion were assessed to possess a mean hydraulic radius of 10 A or less. Doping on the surface leads to the same behaviour, except that the decline in the heat of immersion is followed by an increase in the hi values for the high temperature samples heated at or above 560°C. For the doped samples, formation of a solid solution between Li+ and Mg2+, and a spinel between Al3+ and Mg2+ complicates the results obtained.

Thermal treatment of non-porous silica. The porosity characteristics of silica aerosil TK800 induced by trimethylchlorosilane treatment

Abstract The surface chemistry of the dehydrated parent and silanized TK800 samples were investigated by low temperature (−195.6°C) nitrogen adsorption. For silica aerosil TK800 samples dehydrated at various temperatures, the water content, average pore radii and number of surface hydroxyls were calculated, and the percentage conversion of surface silanols for various dehydrated samples was estimated. As is evident, the availability of surface hydroxyls seems to play a major role in controlling the conversion rates, together with the minor role played by the average pore radius. In general, the conversion of surface silanols is responsible not only for determining the extent of uni-and multi-molecular adsorption, but may also be responsible for some of the observed porosity characteristics.

Effects of thermal treatment on the surface properties of a wide pore silica gel

Abstract A mesoporous silica gel Davidson 59 was thermally treated in vacuo, in the temperature range 20–1000°C. Effects of thermal treatment on the water contents, nitrogen surface areas, pore structure and heats of immersion in water were investigated and discussed. The temperatures selected were 20, 110, 200, 290, 380, 480, 510 and 1000°C. These temperatures were found to cover all the various textural changes resulting from the heat effect. It could be shown that the heats of immersion in water depend primarily on the water content of the sample and are proportional, at least qualitatively to the number of hydroxyl groups on the surface and their availability for interaction with liquid water. The interesting result obtained is that a second factors is involved, namely the pore structure of the adsorbent. A qualitative parallelism exists between the normalized heat of immersion per unit area, and the average pore radius. Apparently the packing of water molecules in narrow pores leads to a decrease in the heat of immersion due to repulsion between the permanent dipoles of the molecules. In narrower pores, the heat of immersion in water is smaller than in wide pores.

Thermal treatment of aerosil 200 silica: Induced surface porosity and surface chemistry relative to the heat of immersion in water

Abstract Silica Aerosil 200 was thermally dehydrated in vacuo in the temperature range 20 – 900 °C. The nitrogen Brunauer-Emmett-Teller (BET) surface areas and the heats of immersion in water are estimated and discussed. The changes in the surface area and the heat of immersion are related to the water content, the number of surface silanols and the average pore radius. All these changes are dependent on the pretreatment temperature. The adsorption studies show that the BET surface area and the reciprocal of the average pore radius are harmonic functions of the pretreatment temperature. The increase in the area of the hysteresis loops in the adsorption isotherms in the temperature range 110 – 480 °C indicates that there is a successive loss of water from the surface and/or the pore system which affects the heats of immersion in water. The heats of immersion in water show two maxima, one at 200 – 300 °C and the other at 480 °C. The number of surface silanols decreases over the temperature range studied. The changes in the heats of immersion are related to the surface chemistry and to the porosity induced by the thermal treatment.

Surface and structural properties of 3, 5 and 10A synthetic zeolites

Abstract The surface and structural properties of synthetic zeolites thermally treated between 100 adn 1000°C were investigated by X-ray, DTA, TG and nitrogen adsorption at −195°C. The zeolites under investigation possess a crystalline structure similar to nepheline; their diffraction patterns completely disappeared on thermal treatment at 800°C due to destruction of the zeolitic lattice. Differential thermal analysis exhibited an endothermic effect centered at about 225°C which may be attributed to dehydration of loosely bound zeolitic water, and two successive exothermic effects; the former was small, centered at 855°C, the latter, which was sharper, appeared at about 920°C. These may be due to evolution of structural water followed by destruction of the zeolite lattice, respectively. Thermogravimetric analysis indicated that the water loss in air exceeded that in vacuum, an effect which is indicative of contraction of the zeolite structure by the action of a high vacuum procedure at elevated temperature. Nitrogen adsorption studies at −195°C were used as a probe to follow the textural and structural changes due to thermal treatment between 100 and 1000°C. Evolution of the monolayer equivalent using the method of Dubinin Radushkevich gave higher estimated values than those calculated by the BET method. These higher values can be attributed to the influence of the micropore filling of nitrogen in the low pressure region which led to distortion in the shape of the isotherms in the initial pressure region, and hence the evaluation of V m by the BET method may be considered unreliable in this connection. In general, monolayer equivalents increase with increase in the pretreatment temperature up to 400°C followed by a gradual decrease as the pretreatment temperature increases. The increase in monolayer equivalents may be correlated with the removal of loosely bound water in the intracrystalline cavities or pores, an effect which leads to an increase in the accessibility of the nitrogen molecule. The decrease in nitrogen uptake on samples thermally treated at temperatures higher than 400°C can be attributed to contraction of the zeolite pores by heating in vacuum: an effect which precedes partial collapse of the structure. The marked decreased obtained on thermal treatment at 800°C may be correlated with destruction of the zeolite lattice producing a mixture of different sintered oxides.

Thermal treatment of Silica Aerosil 200: Heats of immersion in methanol in relation to developed surface porosity and chemistry of the surface

Abstract Silica Aerosil 200 was thermally dehydrated at a set of temperatures in the range 20–1000 °C. The different samples were subjected to measurements of surface area by methanol vapour adsorption and heats of immersion in liquid methanol. Adsorption isotherms for the samples preheated in the range 200–380 °C exhibit hysteresis loops, which may be interpreted in terms of the development of the pore system and/or methoxylation of the siloxane bridges by methanol vapour. In terms of porosity, pore narrowing is responsible for the occurence of hysteresis phenomena in the temperature range 200–380 °C; the high temperature (greater than 380 °C) treatment led to pore widening and consequently to the disappearance of hysteresis loops at higher temperatures. The changes in the Brunauer-Emmett-Teller surface area and the hydroxyl content of silica when they are graphically represented as functions of the pretreatment temperature indicate that the silanol groups act as centres for methanol adsorption. Heats of immersion are characterized by the occurence of two maxima at 200 °C and at 480 °C. The peak located at 200 °C is due to the removal of physisorbed water and the other peak at 480 °C is mainly attributed to methanol interactions with both the silanol groups and siloxane bridges. At low dehydration temperatures (less than 290 °C), the irregular changes in the heats of immersion and the average pore radii when both are graphically represented as a function of the pretreatment temperature are attributed to the different modes of interaction involved in the silanol-molecular water complex at these low temperatures.

Thermal treatment of non-porous silica: Methanol adsorption isotherms for the porosity characterization of oxide surfaces and reference data for methanol adsorption

Abstract The surface chemistry of Degussa Aerosil TK 800 was investigated by measuring the adsorption of nitrogen at the liquid nitrogen temperature and of methanol vapour at 35 °C. The samples were dehydrated at various temperatures in the range 20–900 °C. The resulting changes in the water and/or the hydroxyl contents were found to alter measurably the extent of the interaction at the surface-adsorbate interface, and this appears as a change in the magnitude of the BET C constant. The results of these adsorption measurements for non-porous reference samples were used to construct reference V / V m - P / P 0 curves covering some limited changes in the magnitude of the BET C constant. These reference data are presented and discussed.

Development of surface porosity by trimethylchlorosilane treatment: Nitrogen adsorption measurements on aerosil 200 silica

Abstract Degussa Aerosil 200 silica was thermally dehydrated at a series of temperatures in the range 110 – 510 °C. These preheated samples were then chemically modified with trimethylchlorosilane vapour at room temperature (22 °C). The resulting changes in the hydroxyl content, the percentage uptake and the conversion rates of surface silanols are related to the pretreatment temperature. Nitrogen adsorption measurements were carried out at liquid nitrogen temperature for all the parent and silanized samples. The observed changes in the porosity characteristics are correlated with the conversion rates of surface hydroxyls, the availability of these hydroxyls for the reaction and the average pore radius. It is concluded that the chemical silanization process is not only accompanied by a reaction with surface hydroxyls but may also be responsible for the development of surface porosity.

Thermal treatment of non-porous silica, limited changes in the magnitude of the bet-c constant and their influence of adsorption: reference data for benzene adsorption

Abstract The surface chemistry of Degussa aerosil TK800 is investigated by means of nitrogen adsorption at liquid nitrogen temperature and benzene adsorption at 35°C. Samples are dehydrated at temperatures in the range 20–900°C and subsequent changes in water content (and/or hydroxyl group content) significantly alter the extent of interaction at the surface—adsorbate interface which is shown as a change in magnitude of the BET-C constant. Based on adsorption measurements, non-porous samples are used to construct reference V/V m , P/P o curves which show variable changes in the BET-C constant, namely to 6–35, 40–50 and infinity. Reference curves for benzene adsorption are presented and discussed.

Surface characterization of some selected zinc oxide samples IV: Zinc oxide prepared by the thermal decomposition of zinc basic carbonate

Abstract Zinc oxide samples were prepared by the thermal decomposition of zinc basic carbonate at different temperatures in the range 135 – 500 °C. The arbitrary temperatures were selected using thermogravimetric analysis. The weight lost at these temperatures ( i.e. the isothermal weight loss) was also determined. Nitrogen adsorption measurements were carried out on the different samples; specific surface areas, total pore volumes, average pore radii and other surface parameters were estimated and interrelated. From the thermogravimetric analyses the thermogravimetric losses appeared to be larger than the isothermal weight losses for all the selected temperatures. Contrary to the behaviour expected, this has been interpreted using detailed pore structure studies, an analysis for surface areas located in wide pores and pore volume distribution curves. It is reported that some of the decomposition products (water plus carbon dioxide) are strongly held in micropores (or narrow pores) and this directly influences the estimated surface areas as well as the heat of adsorption (as indicated by the magnitude of the Brunauer-Emmett-Teller (BET) C constant). The fractional volume of micropores and the reciprocal of the average pore radius vary similarly when they are graphically presented as functions of the decomposition temperature; this might be considered indicative of the extent of physisorption of adsorbate molecules. Correspondingly, the similarity between changes in core volume and the slope of the n S - n R plot (if they are graphically presented as functions of the decomposition temperature) is a tentative measure of the availability of physisorbed molecules to be chemisorbed. The sequence of these two processes is related to the decomposition mechanism of zinc basic carbonate. It was found from this investigation that the zinc oxide samples possess high surface areas accompanied by low BET C constants. The chemisorption process of different alcohols is primarily controlled by the volume of cores. The catalytic dehydrogenation reactions of alcohols is controlled by the extent of the surface, as well as the capability for chemisorption; all these variables are controlled by the thermal treatment conditions.