Internal Silanol Groups Research Articles

Dealumination of Zeolite β via Dicarboxylic Acid Treatment

It is demonstrated that zeolite β and zeolite β containing catalysts can be dealuminated to very low acidity levels using a novel oxalic acid treatment without reducing zeolite integrity. The effect of the oxalic acid treatment has been studied over a wide range of treatment conditions for both silica-bound and unbound zeolite β catalysts. Greater than 90% dealumination is observed with a concomitant reduction in n-hexane-cracking activity as measured by the alpha (α) test. Removal of framework aluminum occurs via a two-step hydrolysis/chelation mechanism, with the oxalic acid acting both as an acid and as a chelating agent. Framework aluminum removal is accompanied by the formation of internal silanol groups. Water soluble aluminum oxalates are present in the extracted solutions. Silanol groups are annealed with extended oxalic acid treatment. Oxalic acid treatment results in a unique contraction of the zeolite β lattice structure not observed for mineral acid treated or steamed zeolite β catalysts.

Interaction of water with the zeolite HY, studied by FT i.r.

The interaction of water with the Bronsted acid sites of zeolite HY at 25 and 80 °C, and subsequent decomposition of HY upon thermal desorption of the water, has been studied by FT i.r. If less than one molecule of water was sorbed per Al atom in the zeolite, the acid sites were fully restored after water desorption. When the amount of sorbed water was increased, permanent loss of Bronsted acidity occurred on water desorption, along with collapse of the zeolite framework. Upon water sorption, the formation of new i.r. hydroxyl peaks was observed at 3710, 3675, and 3615 cm−1. These may be due to partial removal of Al from the lattice to form AlOH species, accompanied by the formation of internal silanol groups.

Calorimetric study on the state of C1–C4 alcohols sorbed on silicalite

Differential molar heats of sorption have been determined calorimetrically for methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol and 2-methylpropan-1-ol on silicalite as a function of pore filling . The results suggest that the first alcohol molecules sorbed interact wtih lattice defects; as the number of alcohol molecules rises, they are sorbed on the regular silicalite framework. A comparison of the heat of sorption curves of methanol, propan-1-ol and propan-2-ol at low pore fillings with that of triethylamine on silicalite supports the view that most of the defects in the silicalite lattice consist of internal silanol groups which predominantly occur as vicinal pairs. From a comparison of the heat of sorption curves of the alcohols studied with those of alkanes and diethyl ether, it is concluded that a substantial contribution to the heat of sorption of the alcohols on the regular silicalite framework stems from hydrogen-bonding interactions of the alcohol molecules with framework oxygen atoms and/or with each other.

Diffusion of ethane in silicalite-1 by frequency response, sorption uptake and nuclear magnetic resonance techniques

The diffusion of ethane in ZSM-5 has been studied using four different experimental methods: a frequency-response technique, a single-step frequency-response method (an improved sorption uptake–desorption technique), an n.m.r. pulsed field gradient spin–echo technique and, finally, an n.m.r. tracer desorption technique. Large and uniform silicalite-1 crystals were prepared and used in all four techniques. Different concentrations of tetrapropylammonium ions in the synthesis batch alter systematically both the crystal habit and the relative amounts of internal silanol groups in the silicalite-1 framework, indicating different concentrations of defect Si—O—Si bonds. For the genesis of the defective siloxane bonds, a model has been presented. Comparing the translational mobility of ethane on a selected sample, the diffusion coefficients measured by the two n.m.r. methods were found to be ca. 150 times larger than the corresponding coefficients derived from the frequency response and sorption uptake–desorption methods. This experimental finding is explained by crystal bed depth influences on the molecular uptake in the latter sorption experiments.

ChemInform Abstract: Investigation of Internal Silanol Groups as Structural Defects in ZSM‐5‐type Zeolites.

AbstractFrom 1H MAS NMR investigations it is concluded, that synthesis of HZSM‐5‐type zeolites by means of the TPA cation as a template leads to a high concentration of lattice imperfections in the form of non‐intact Si‐O‐Si bonds.

Investigation of internal silanol groups as structural defects in ZSM-5-type zeolites

A 1H Magic-angle spinning nuclear magnetic resonance investigation of large and uniform crystals of HZSM-5 type zeolites, synthesized either by means of TPA or without organic templates, shows that using TPA leads to a high concentration of defect centres within the ZSM-5 structure. Owing to non-intact Si—O—Si bonds, up to 8% of the lattice Si were found to be present as Si—OH groups. A comparison of the 1H—29Si cross-polarization spectra with the variation of the 1H n.m.r. chemical shift of the Si—OH groups as well as with the results of ion-exchange experiments suggests a vicinal arrangement of the silanol groups. High concentrations of such structural defects within the crystals lead to a slight decrease of crystal sorption capacity for n-hexane and significantly reduce crystal hydrophobicity. Both the intracrystalline self-diffusion and the molecular reorientation of sorbed molecules as measured by means of the 1H n.m.r. pulsed field gradient technique and 13C n.m.r. lineshape analysis, respectively, show an enhanced molecular mobility of guest molecules within the defective ZSM-5 structure