The very first stages of the incorporation of tetraethoxysilane (TEOS) into aqueous tetrapropylammonium hydroxide (TPAOH) by hydrolysis are investigated to clarify the formation of silicate species in solution: oligomers and nanoparticles. Silicate speciation of both oligomers and nanoparticles were characterized using quantitative 29Si NMR, electrospray ionization mass spectrometry (ESI-MS), dynamic light scattering (DLS), and 1H diffusion-ordered NMR spectroscopy (DOSY). The main parameter measured for following this formation with the advancement of hydrolysis of TEOS is the evolution of silicon connectivity, in oligomers as well as in nanoparticles. At the beginning of TEOS hydrolysis, small oligomers are formed, which grow in number and size as the reaction progresses, with an average connectivity going from 0 to 2.1. At Si/TPAOH ratio of 1.0 and above, nanoparticles form through aggregation of oligomers with some additional condensation. Their connectivity varies from 2.4 to 3.1. These nanoparticles cannot be confused with condensed silica particles.

Silicon-29 longitudinal (T 1 ) and transverse (T 2 ) NMR relaxation times have been measured in the clear solution precursor of silicalite-1 of composition 25 TEOS-5 TPAOH-400 H 2 O. The nanoparticles as well as the silicate oligomers are giving rise to observable resonances. An unusually long T 1 relaxation time of 126 s is observed for Q 4 in nanoparticles. Proper care for acquisition is therefore required for quantifying the distribution of Q n of the nanoparticles, an essential measurement to follow the nanoparticles connectivity evolution. IntroductionUnderstanding how zeolites nucleate and grow is crucial to optimizing catalytic and molecular-sieving properties of these important materials, especially when tailoring crystal size and shape. In this study, silicalite-1 is synthesized from a clear mixture of tetraethoxysilane (TEOS), tetrapropylammonium hydroxide (TPAOH) and water, providing a model system for zeolite formation. The mixture is actually a colloidal suspension of nanoparticles in water-ethanol with many coexisting silicate oligomers. A comprehensive silicate speciation is far from being completed 1,2 and the existence of some structures such as the double-five ring, a key unit for mobile type five (MFI) topology, is still not evidenced. 3,4 Even though the presence of the nanoparticles as an amorphous-like structure is agreed upon, their precise role in the synthesis remains a subject of debate. [5][6][7][8] In particular, the question of whether these nanoparticles contain occluded TPA templates or are simply TPA-free silica entities, is still open. 9,10 Even though a core-shell structure, 11 with a silica core surrounded by a shell of organocations, appears now to be the most accepted description, the chemical composition, as well as the local structure are not elucidated yet, due to a lack of an appropriate characterization technique. For instance, X-ray diffraction (XRD) cannot be used because of the lack of periodicity and the small size of particles below the coherence domain 12 and 29 Si NMR provides very poorly resolved spectra with severe signal broadening. 13-18 Nevertheless, valuable information on internal Si connectivities can be provided by quantification of 29 Si Q n distribution, especially the Q 4 peak, 13,17,18 since it influences strongly the interpretation on the internal degree of Si condensation. In a recent paper, Vlachos et al. have pointed out that a much longer recycle delay than the usual values considered for silicate solutions containing nanoparticles, is to be used for Q 4 resonances. 18 The spectra were however of