Gamal M.S. El Shafei

Interaction between molybdena and silica: FT-IR/PA studies of surface hydroxyl groups and pore structure assessment

Silica impregnated in aqueous solutions of ammonium heptamolybdate to achieve different Mo loadings (3–15 wt.%) is studied by the Fourier transform infrared/photoacoustic (FT-IR/PA) technique. Special attention is given to the high frequency region of surface hydroxyl groups. Air calcination at 500°C of the Mo loaded silica caused the evolution of defined features at 3720 and 3550 cm−1 which are attributed to the free and hydrogen-bonded components of a pair of hydrogen-bonded silanol groups. The presence of such silanols on the surface of silica heated at 500 °C is attributed to the hydrolysis of open siloxane bridges by the action of constitutional water that is liberated from the bulk interior due to heating at 500 °C. The open siloxane represents a site of weakly attached Mo species that escape from the surface upon heating. These detached species are seen as the precursor of MoO3 detected on the surface (band at 994 cm−1). A surface polymolybdate phase is also detected (band at 970 cm−1) and considered to result from condensation between Mo species adsorbed on the surface. The effect of fluorination on the interaction between Mo and silica, and the pore structure of the Mo loaded silica are also discussed.

TREATMENT AND HALOGENATION ON LOW MOLYBDENUM SILICA: DIFFUSE REFLECTANCE IR FOURIER TRANSFORM STUDY (DRIFTS)

Abstract Low molybdenum silica (1.2 wt.% molybdenum) has been prepared by impregnation (pore filling method) from an aqueous (NH 4 ) 6 Mo 7 O 27 solution (pH ≈ 6). The effect of temperature (373–673 K) was followed by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The interaction between molybdenum species, MoO 4 2− , and silica surface (OH pairs) is recognized at 373 K through the appearance of a band at 905 cm −1 attributed to an MoOSi linkage, that shifted, and decreased in intensity, with the increase in temperature to appear at 915 cm −1 at 673 K. Bands at 555 ( ν s Mo  O  Mo ) and 881 cm −1 ( ν as Mo  O  Mo ) are correspondingly developed with temperture, and only at 673 K a band due to MoO 3 (998 cm −1 ) is recognized. These observations are used to propose a firm correlation between the detached molydenum species upon the breaking of the MoOSi bonds and MoO 3 formation. The spectral changes in the stretching frequency region of surface hydroxyls indicated that as the separation between adjacent hydroxyls (involved in interaction with MoO 4 2− ) increases, the strength of the MoOSi link decreases. The presence of Cl − or F − on the surface of silica was found to decrease the extent of interaction between the molybdenum species and silica through decreasing the probability of finding vicinal hydroxyls. The formation of polymolybdate phase identified on the surface of molybdenum-supported silica is also inhibited by halogenation.

Structural and Textural Properties of Perchlorated and Persulphated Mixed (Hydr)Oxides of Zirconium and Titanium

Perchlorated and persulphated mixed hydroxides of zirconium and titanium were prepared by coprecipitation and impregnation in aqueous HClO4 or (NH4)2S2O8 solutions of 0.05, 0.10, 0.20 and 0.40 M concentrations. An alternate sequence of impregnation followed by calcination or vice versa was conducted and the samples obtained studied using XRD, FT-IR, pyridine titration and low-temperature (–196°C) nitrogen adsorption methods. XRD indicated that the presence of titanium stabilized the tetragonal modification of zirconia and almost completely prevented the usual tetragonal → monoclinic transformation upon calcination at 650°C. Both S2O82– and ClO4− anions at their lowest concentration level (0.05 M) partially retarded the crystallization which occurred upon calcination at 650°C in their absence. However, the two anions showed different effects. Whereas the perchlorate anion prevented the formation of a crystalline titania phase (anatase) to a greater extent than that of crystalline zirconia, the persulphate anion showed the opposite effect. Complete inhibition was observed with both anions at a concentration of 0.4 M. This effect was attributed to adsorption of the anions on the hydroxy species of zirconium and titanium formed initially, as demonstrated by IR spectroscopy which showed that the anions were of lower symmetry, viz. C2v, due to their bonding to the hydroxy species. Calcination at 650°C caused the material formed initially to lose virtually all its initial high surface area because of crystallization. The prevention of crystallization by added anions was reflected in the retention of a relatively high surface area even after calcination at 650°C. The recorded difference in the interactions of the anions with the hydroxy species formed initially was also reflected in the texture of the anion-modified solids. The protecting influence of the ClO4− anion increased with its increasing concentration in the system, whereas the corresponding effect with the S2O82– anion increased up to 0.10 M concentration and then decreased at higher concentrations.

Titania-induced changes of silica texture upon moderate heating

Abstract Induced changes in the texture of silica gel due to physically mixed titania (15–85 mol%) upon heating at 300°C or 600°C in air are inspected through a detailed pore structure analysis using adsorption of N2 at 77 K. The decreases in surface area and pore volume with the corresponding variations in the average hydraulic pore radius of silica recorded at both heating temperatures are attributed to the disturbance of the normal conditions of silica dehydration–dehydroxylation due to presence of titania. Water desorbed from titania upon heating up to 300°C alters the arrangement of siloxane links that form between primary particles of silica upon dehydroxylation. The active Lewis sites of titania surface that develop in the course of heating up to 600°C disturb the usual equilibrium between surface and bulk water of silica.

Change of Structural and Adsorption Properties Due to Isomorphous Substitution in Hydrotalcite-like Materials

Samples of hydrotalcite-like compounds with the general formula [M2+1-xM3+x(OH)2]x+•Ax–•nH2O were prepared via coprecipitation through the addition of NaOH to mixed chloride salt solutions. On maintaining the M2+/M3+ ratio equal to three, it was possible to effect isomorphous substitution. This led to the preparation of different compounds which could be compared to the parent material in which M2+ = Ni and M3+ = Al. Other divalent cations used were Mg2+ and Zn2+ with Cr3+ and Fe3+ being employed as trivalent cations. The ratio between cations of the same charge was unity in all the isomorphously substituted samples and substitution was conducted for one oxidation state only at a time. Hence, all samples contained a maximum of three different cations. The charge compensating anion in the interlayers, A, was identified as mainly CO32– with the co-existence of Cl− anions to different extents. The structural and textural properties developed as a result of the substitutions effected were studied by XRD, IR, DTA, acid–base titration and mercury porosimetry measurements. Various samples were used for the adsorption of 4-chlorophenol to examine the effects on the adsorption properties arising from isomorphous substitution. The major effect having a marked influence on the adsorption properties arose from the nature of the compensating anion in the interlayers. Different tendencies of different cations to be accommodated in the layer structure resulted in different interlayer spaces and variable conformations of the charge compensating anion. In all cases, at low equilibrium concentrations, the adsorption of 4-chlorophenol (pH = 11) occurred at the edges of the layer structure accompanied by Cl−/OH− exchange. The increasing order in the amount of 4-chlorophenol adsorbed was matched by an increasing order in the interlayer space due to substitution. At higher values of the equilibrium concentration, the adsorption of both phenolate and OH− anions decreased. This was because such adsorption into the interlayer space to replace the CO2– anion was diffusion-controlled, making the process improbable at high pH values.

Effect of mechanically mixed ZnO on the evolution of pore structure of SiO2 upon thermal treatment

N2 adsorption at 77 K was used to follow the change in the pore structure of silica (mesoporous) produced on heating at 300 and 600‡C in the presence of different contents of mechanically mixed ZnO (15–85 mol%).Heating of pure silica at 300‡C caused pore narrowing. This proceeded differently in the presence of ZnO, the pore system being split into two ranges of limited sizes. The pore widening effect upon heating silica at 600‡C was greatly reduced upon the addition of ZnO, to the extent of the evolution of some micropores at low levels of ZnO.At both heating temperatures, the composition 30% ZnO represented a transient state through the flux of pore system change. The IR spectra indicated that at this composition the silica particles are more oblate.The pore structure variations due to heating at the examined temperatures correlated with the solid-state reactions that take place at 1000‡C, where the spinel Zn2SiO4 is detected.

Solid Acids from Persulphated and Perchlorated Physical Mixtures of Zirconium and Titanium Hydroxides

Calcination at 650°C of a physical mixture of zirconium and titanium hydroxides led to the formation of the corresponding oxides, monoclinic zirconia and anatase. The adsorption of perchlorate or persulphate anions (as 0.05, 0.1, 0.2 or 0.4 M aqueous solutions) before calcination did not inhibit crystallization; however, perchlorate anions activated the formation of rutile in addition to the predominant anatase phase. Indeed, the adsorption of perchlorated anions prior to calcination allowed the thermodynamically less stable tetragonal phase of zirconia to be detected in addition to monoclinic zirconia at ambient temperature. In contrast, the adsorption of persulphate anions before calcination stabilized the tetragonal phase with no rutile phase being detected in this case. Infrared spectroscopy showed that adsorbed S2O82– anions were held more strongly by the solid than ClO4− anions which tended to decompose when the solid was calcined. The acidities of the solid acids produced because of S2O82– or ClO4−anion adsorption were studied via the adsorption of pyridine (pKa = 5.3) from cyclohexane solution. The amounts and strengths of the acid sites formed during persulphate treatment were higher than those resulting from perchlorate adsorption. The strength of the acid sites formed on samples calcined before loading with S2O82– or ClO4− anions showed no significant differences. Variations detected in the structural aspects arising from S2O82– or ClO4− anion adsorption were reflected in the texture as assessed by nitrogen adsorption at −196°C.