Microporous nitrides

The TPR method has now been applied to TlN O [52], VN [23], and NbC [53]. Although the TPR method produces high surface area materials, the pore structure of these is usually not controllable. Often, the pores are in the micropore regime (< 3 nm). However, a number of the solid state transformations that lead to carbides and nitrides are topotactic and exhibit pseudomorphism (retention of external particle size and shape) This provides a possible means of engineering the pore structure by preparing oxide precursors with the desired external morphology. 

In addition to aluminosilicates, crystalline microporous materials can be phosphate-based. The aluminophosphate (A1P04) framework is electroneutral (analogue of Si02), and the aluminum and/or phosphorus tetrahedral atoms can be substituted by a number of metal and non-metal atoms that result in producing charged frameworks [1-3], e.g. Si+4 substitution for P+5. In addition, numerous other metal oxide, and nitride based microporous materials have been reported recently [4, 5]. 

Textural properties of the starting and nitrided zeolites are presented in Table 1. The increase of the nitridation temperature causes a decrease in Vmjcroj Vt and Sbet, while Vmeso and Sext increase slightly. Thus the increase of the nitridation temperature leads to the collapse of part of the microporous structure, to the formation of mesopores and consequently to the extension of the "external" surface. The Vj decreases at the same time as Vmicro> indicating that the fall of Vmicrois not Compensated by the rise of Vmeso- No correlation was found between these results and XRD, except in the case of the sample nitrided at 900°C for which a decrease of crystallinity occurs simultaneously with an important loss of Vmicro-... 

In spite of the increase of the nitridation time from 48 to 72 h, these two samples show practically the same isotherm and thus the same textural properties (Table 1). However, the sample nitrided during 96 h is characterized by a lower micropores volume, which may be a consequence of the partial deterioration of the zeolitic structure after the prolonged treatment under ammonia flow. The partial deterioration of the zeolitic structure is confirmed by the loss of crystallinity revealed by XRD. 

Iron and its compounds (carbide, nitride), as well as ruthenium, cobalt, rhodium, and molybdenum compounds (sulfide, carbide), are used most frequently to produce high-molecular-weight hydrocarbons. Iron can be prepared as a high-surface-area catalyst (==300 m /g) even without using a microporous oxide support. 7-AI2O3, Ti02, and silica are frequently used as supports of the dispersed transition-metal particles. Recently zeolites, as well as thorium oxide and lanthanum oxide, have... 

The preceding discussion of inorganic microporous structures illustrates the great variety of metal eations that may be accommodated in porous frameworks. By contrast, relatively little progress has been made in the preparation of porous frameworks with anions other than oxide ions. The obvious candidates are nitrides and sulfides and there have been some reeent advances in these directions. 

Because of the uniquely ordered structure of template-synthesized porous carbons, they themselves can be used as templates to replicate other materials with an ordered porous structure that is difficult to make using traditional methods [121,338-348], Thus, CMK-3 carbon was first demonstrated as a template to prepare mesoporous silica [121,338]. An OMC prepared from an MCM-48 template was used to prepare nanostructured silica [339]. Nanocasting of CMK-3 using ZSM-5 crystals yielded a mesoporous zeolite with both mesopores and micropores [340]. More recently, the preparation of a novel class of mesoporous aluminosilicate molecular sieves was described [341]. The preparation of mesoporous boron nitride (MBN) and mesoporous carbon nitride (MBCN) with very high surface area and pore volume was recently realized using a well-ordered hexagonal mesoporous carbon as a template and boron trioxide as a boron source [342]. Nonspherical silica nanocases with a hollow core and mesoporous shell were also produced [343]. 

Brow R.K., Pantano C.G. Thermochemical nitridation of microporous silica films in ammonia. J. Am. Ceram. Soc. 1987 70 9-14... 

The decomposition under partial pressure of ammonia of Mo(CO)g loaded in EMT zeolite by chemical vapor deposition and subsequent nitridation under flowing ammonia were used to prepare encaged molybdenum nitride clusters [263]. XPS measurement showed that molybdenum nitride was formed at a nitridation temperature as low as 723 K TEM measurements indicated that the nitride phase was homogeneously dispersed in the zeolite micropores as clusters smaller than 1 nm. 

---