Methanol-silica gel system. II. Molecular diffusion and proton exchange from pulse proton magnetic resonance data

Abstract

six polar hydroxy sites. Sorbitol, in contrast, appearsAcknowledgmerti. The financial assistance of the to be a struchre breaker, as is indicated by the mono-National Science Foundation is gratefully aeknowlronic negatiw erilhalpies over the entire concentration edged. J. K. S. wishes to thank Professor Y, Marcus range investigated, This behavior of ART, is similar at> the Hebrew University for his hospitality. for XaCE in acjil~aoixs hydrogen peroxideJ5 ureaJ6 and %,hi& differ d r o~i d e s .~ in position of one of their six hy-bility of Inorganic and Organic Compounds,'' Supplement to 3rd Publication costs assisted by the Muage Royal de E'Afrigue CentraleThe proton motions in the methanol-silica gel (Xerogel) systems have been deduced from the measurement of the spin-lattice (TI) and spin-spin (T,) proton relaxation times in the systems C€LOD-XOD, CDIOH-XOK, and CH,OH-XOH, where XOH and XOD stand for the Xerogel-OH or Xerogel-OD surfaces, respectively. In CH30D-XOD, only one TI or T2 is observed. The experimental data may be accounted for by a laandational motion of CH,OD, the methyl. group rotating very rapidly around the triad axis. In CD30H-XOH, one TI aiid two Tt are measured. TI is the superimposition of two functions. The rate of exchange of nuclei between the two populations is fast with respect to T1-l but slow with respect to T2-I. The long TZ represents the diffusion motion observed in the first system, while the short T2 is assumed to be due to a proton exchange process between the hydroxylic protons of C&OH and XOH or between CDIOI~ molecules. Moreover, it has been possible to compute the observed Ti for the CHIOH-XOH system from the data obtained for the two simpler cases. At 25", the surface diffusion coefficient of adsorbed methanol is reduced by two orders of magnitude with respect to that in the liquid for a degree of coverage e ^I 0.6. For 1 < e < 2, the lowering is by a factor of 10. For e 2 0.7, the surface diffusion coefficient at 25' is the same as that in the liquid state at -85". More than three layers should be probably required to find a diffusion coefficient in the adsorbed state close to that in the liquid state. In the assumed exchange process a proton is transferred from the surface into a cluster of adsorbed species where it jumps from a molecule to another before coming back on the surface: [-OH + CB301-I 3 1 0-+ CH10H2+; CH30H2+ + CH80J1 -+ CH30H + C€InOIh+; CH3OH2" + -0-[ -+ ] -OH + CHaOH. The ratio of the exchange to the molecular diffusion jump frequency ( v e / u d ) is 6 x 10-3 for e = 1.7 and 2.5 x 10-2 for e = 0.8. This means that the number of times a methanol molecule is protonated during its stay on an adsorption site increases at decreasing degree of coverage. For 0 = 0.8, u./ud is very close to the value observed in the liquid protonated methanol al, 25" The activation energies of both the diffusion and the proton exchange processes are dose to each other (-6 kcal at 8 = l), and there are approximately equal to half the isosteric heat of adsorption. Both these motio...

Atomic scale characterization and surface chemistry of metal modified titanate nanotubes and nanowires

“…Conductivity was found to increase with the dipole moment of the adsorbed species in Vycor-glass [107]. Experimental proof for proton exchange between surface functional groups and adsorbed molecules was obtained in CH 3 OH/SiO 2 [136], water/SBA-15 [137] and various water/ sulphated polystyrene [138] systems. The exchange was heavily dependent on surface coverage [135,136] and one adsorbate could be protonated multiple times while being anchored at a specific adsorption site.…”

“…Experimental proof for proton exchange between surface functional groups and adsorbed molecules was obtained in CH 3 OH/SiO 2 [136], water/SBA-15 [137] and various water/ sulphated polystyrene [138] systems. The exchange was heavily dependent on surface coverage [135,136] and one adsorbate could be protonated multiple times while being anchored at a specific adsorption site. The importance of functional groups in conductive processes was experimentally confirmed in various amine-intercalated titanium(IV)-phosphate [139] and zirconium hydrogen phosphate [140][141][142] systems.…”

Water Adsorption at Pyrogenic Silica Surfaces Modified by Phosphorus Compounds

“…The NMR method is widely used for study of structures of the active surface sites and adsorption complexes on silica. − The separated signals of the ⋮SiOH and Si(OH) 2 groups at the silica surface were observed by the 29 Si NMR high-resolution method. , The translational and rotational mobility of the adsorbed molecules of different natures was studied by measurement of the longitudinal and cross relaxation of the adsorbed compounds. , Adsorbed phosphorus compound structure may also be studied by the 31 P NMR method. , In addition, the 1 H NMR method allows us to investigate the adsorbed water layer characteristics and their influence on the surface groups and other adsorbed compounds.…”

“…The NMR method is widely used for study of structures of the active surface sites and adsorption complexes on silica. [7][8][9][10][11] The separated signals of the tSiOH and dSi-(OH) 2 groups at the silica surface were observed by the 29 Si NMR high-resolution method. 7,8 The translational and rotational mobility of the adsorbed molecules of different natures was studied by measurement of the longitudinal and cross relaxation of the adsorbed compounds.…”

“…7,8 The translational and rotational mobility of the adsorbed molecules of different natures was studied by measurement of the longitudinal and cross relaxation of the adsorbed compounds. 9,10 Adsorbed phosphorus compound structure may also be studied by the 31 P NMR method. 4,11 In addition, the 1 H NMR method allows us to investigate the adsorbed water layer characteristics and their influence on the surface groups and other adsorbed compounds.…”