Zeolite synthesis molecular water

Microwave Synthesis of Zeolites and Molecular Sieves The use of microwaves holds promise for efficiency improvements in zeolite synthesis due to the rapid heating possible when using microwave radiation [166], The first report of microwave synthesis of zeolites was by Mobil Oil in 1988, which broadly claimed the synthesis of zeolite materials in the presence of a microwave-sympathetic material, such as water or other pro tic component [167]. A number of reports have appeared since, including synthesis of zeolites Y, ZSM-5 [168] and metaUoaluminophosphate-type materials, such as MAPO-5 [169], There have also been extensive investigations in using microwaves for zeoHte membrane synthesis. Recent reviews discuss the progress in microwave zeoHte synthesis [170, 171]. 

Water and hydroxyl ion are the classic mineralisers in hydrothermal synthesis, firstly because aqueous alkali dissolves amphoteric oxides and so promotes mobility and mixing of molecular and ionic species as a pre-requisite for reaction. A second vital role is that of molecular water which (see below) stabilises aluminous zeolites by filling channels and cavities. This role can be shared or taken over by organic molecules (e.g. in porosils, silica-rich zeolites or AlPO s), and by salts (e.g. in scapolites, sodalite and cancrinite). 

Metal oxide catalytic materials currently find wide application in the petroleum, chemical, and environmental industries, and their uses have significantly expanded since the mid-20th century (especially in environmental applications) [1,2], Bulk mixed metal oxides are extensively employed by the chemical industries as selective oxidation catalysts in the synthesis of chemical intermediates. Supported metal oxides are also used as selective oxidation catalysts by the chemical industry, as environmental catalysts, to selectively transform undesirable pollutants to nonnox-ious forms, and as components of catalysts employed by the petroleum industry. Zeolite and molecular sieve catalytic materials are employed as solid acid catalysts in the petroleum industry and as aqueous selective oxidation catalysts in the chemical industry, respectively. Zeolites and molecular sieves are also employed as sorbents for separation of gases and to trap toxic impurities that may be present in water supplies. Significant molecular spectroscopic advances in recent years have finally allowed the nature of the active surface sites present in these different metal oxide catalytic materials to be determined in different environments. This chapter examines our current state of knowledge of the molecular structures of the active surface metal oxide species present in metal oxide catalysts and the influence of different environments upon the structures of these catalytic active sites. 

Querol et al. [25] have presented an overview on the methodologies for zeolite synthesis from the fly ash. The authors have detailed the conventional alkaline conversion processes, with special emphasis on the experimental conditions to obtain high cation exchange capacity (CEC) zeolites. They have reported that zeolitic products having CEC up to 300 meq./lOO g, can be obtained from high-glass fly ash by direct conversion and the main application of this material is the uptake of heavy metals and ammonium ion from polluted water. It has been clarified that some of the zeolites synthesized, are useful as molecular sieves to absorb water molecules from gas streams or to trap SO2 and NH3 from low water gaseous emissions based on their pores and molecular sizes as depicted in Fig. 7.4. 

About one decade ago Bass et al. [13,14] proposed first that such approach could help in exploring the structure of water dissolved silicates. Following this initiative, recently we critically evaluated how the published FTIR and Raman assignments could be adopted for differentiating between the molecular structures of some commercially available sodium silicate solutions [7-9,15], In this paper we present comparative structural studies on aqueous lithium and potassium silicate solutions as well. According to some NMR studies, the nature of A+ alkaline ion and the A+/Si ratio barely affects the structural composition of dissolved silicate molecules [5], In contrast, various empirical observations like the tendency of K-silicate solutions to be less tacky and more viscous than their Na-silicate counterparts, the low solubility of silica films obtained from Li-silicate solutions compared to those made from other alkaline silicate solutions, or the dependence of some zeolite structures on the nature of A+ ions in the synthesis mixture hint on likely structural differences [16,17]. It will be shown that vibrational spectroscopy can indeed detect such differences. 

Along with hydrophobicity, large amounts of both water (to promote hydrolysis) and methanol employed as co-solvent in the catalyst preparation (to promote homogeneity) are needed to ensure optimal reactivity, showing the number of experimental parameters of the sol-gel synthesis which can be controlled independently to optimize the performance of the resulting catalyst. Finally, in contrast to zeolites and other crystalline porous materials, amorphous sol-gel materials show a distribution of porosity which does not restrict the scope of application of sol gel catalysts to substrates under a threshold molecular size. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

In 1978, the same year that the structure of ZSM-5 was first described, Flanigen and her co-workers reported the synthesis, structure and properties of a new hydrophobic crystalline silica molecular sieve (Flanigen et al., 1978). The new material, named Silicalite (now generally called Silicalite-I), has a remarkably similar channel structure to that of ZSM-5 but contains no aluminium. It was pointed out by the Union Carbide scientists that, unlike the aluminium-containing zeolites, Silicalite has no cation exchange properties and consequently exhibits a low affinity for water. In addition, it was reported to be unreactive to most acids (but not HF) and stable in air to over 1100°C. 

Alternatively, surface-mediated synthesis involves the immobilization of a mononuclear metal complex (similar to the ones described in Sect. 19.2) and the subsequent treatment in CO at different temperatures to form a supported metal carbonyl cluster, for example, [HIr (CO)jJ formed from Ir(CO)2(acac) on MgO and [Rhj(CO),5] formed from Rh(CO)2(acac) on MgO and y-Al Oj [45 7]. Synthesis of metal carbonyl clusters on oxide supports apparently often involves OH groups or water on the support surface analogous chemistry occurs in solution [13]. The synthesis of molecular catalysts from a mononuclear metal complex is likely to occur with a yield less than that associated with simple adsorption of a preformed metal cluster, and so the latter precursors are preferred, except when they do not fit into the pores of the support (e.g., a zeolite). 

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