Zeolites acidity synthesis

Many standard reactions that are widely applied in the production of fine chemicals employ. strong mineral or Lewis acids, such as sulphuric acid and aluminium chloride, often in stoichiometric quantities. This generates waste streams containing large amounts of spent acid, which cannot easily be recovered and recycled. Replacement of these soluble mineral and Lewis acids by recyclable. solid acids, such as zeolites, acid clays, and related materials, would represent a major breakthrough, especially if they functioned in truly catalytic quantities. Consequently, the application of solid acids in fine chemicals synthesis is currently the focus of much attention (Downing et al., 1997). 

Modulation of zeolite acidity by post-synthesis treatments in Mo/HZSM-5 catalysts for methane dehydroaromatization... 

Over iron-phthalocyanine encaged in zeolite Y and using tertiary-butylhydroperoxide (t.-BHP) as oxidant, even cyclohexane can be converted to adipic acid. Selectivities of up to 35 % at conversions around 85 % have been reported. Unfortunately, however, a reaction time of 33 hours at 60 °C was required to achieve this conversion. Although the activity of the latter catalyst is certainly much too low to compete with the conventional catalytic systems for adipic acid synthesis, it provides interesting prospects for further developments. For the near future, we perceive that more and more groups will be working in this interesting field of catalysis by zeolite inclusion compounds. 

The concentration of acid sites was determined by temperature programmed desorption (t.p.d.) of NH3. The degree of ion exchange was calculated from the difference in concentration of the strong Brensted acid sites present before and after ion exchange. The structure of the zeolites after synthesis or postsynthetic modification was verified by XRD. 

The control of zeolite acidity is of special importance when catalyzing reactions involving strong bases such as NH3 or pyridines. For such reactions a zeolite catalyst with excessive acidity will be rapidly poisoned by adsorption of the reactant or the basic products. For instance, in the aldol condensation of aldehydes and ketones with ammonia for the production of pyridine and 3-methylpyridine, an important intermediate in the synthesis of vitamin B3, milder acidities are preferred [11]. 

Recently, there has been considerable interest in the isomorphous substitution of tetrahedral aluminium in zeolite frameworks with catalytically active elements such as iron, gallium and boron. These materials have acidities Afferent from the corresponding aluminosilicates leading to altered activity, selectivity and stability. Mdssbauer spectroscopy has been used to study the iron incorporated into zeolites during synthesis. Fe(III) can be present on tetrahedral framework sites as Fe " cations acting as counterions and as Fe(III) oxides precipitated in or on the zeolite crystals. The most common iron oxide is a-Fe203 which contains iron only in octahedral coordination. 

Even though the catalytic systens described iu the preceding sections provide greater practicality than stoichiometric systens, their application in industrial synthesis is not widespread. However, in more recent times, maximum effort has been directed toward the use of solid acid catalysts. In fact, heterogeneous catalysts can be easily separated from the reaction mixture and reused they are generally not corrosive and do not produce problematic side products. Different classes of materials have been studied and utilized as heterogeneous catalysts for Friedel-Crafts acylations these include zeolites, (acid-treated) metal oxides, and heteropoly acids (HPAs) already utilized in the hydrocarbon reactions [24]. Moreover, the application of clays, perfluorinated resin-sulfonic acids, and supported (fluoro) sulfonic acids, mainly exploited in the production of fine chemicals, is the subject of intensive studies in this area. 

Methyl acetate forms as an intermediate in the acetic acid synthesis from methanol and carbon monoxide and is used as a solvent for cellulose, nitrates, esters and ethers. When this ester is produced via esterification of acetic acid with methanol, generally mineral acids like sulfuric acid, hydrogen chloride and aryl sulfonic acid (such as p-toluenesulfonic acid) are used as homogenous acid catalysts, while cation exchange resin and zeolites are used as heterogeneous catalysts. The relevant literature on this subject is summarized in Chapter-1. Some important studies on esterification of acetic acid with methanol are summarized below. 

Except for cobalt systems, other metals also demonstrate activity in the Fischer-Tropsch process. Mo/HZSM-5 turned out to be active in FT synthesis [98], The catalysts were tested at a low H /CO molar ratio 1.0, this composition being typical for biomass gasification. Liquid hydrocarbons obtained on Mo/HZSM-5 at 573 K were presented by alkylaromatics and lower branched and cyclic alkanes. The formation of aliphatic hydrocarbons was close to zero. The gas products included Cj-C alkanes. Higher alcohols and carboxylic acids (C,-Cg) were observed in the aqueous phase. The formation of hydrocarbons on Mo/zeolite is accounted for by the bifunctional zeolite acidity and molybdenum metal activities via alcohols as intermediates. The zeolite Y was also found to be a good support for Mo in the FT reaction. 

Garcia-Trenco, A. Martinez, A. Direct Synthesis of DME from Syngas on Hybrid CuZnAl/ZSM-5 Catalysts New Insights into the Role of Zeolite Acidity. Appl Catal A Gen. 2012,411 12, 170-179. 

The isomorphic substituted aluminum atom within the zeolite framework has a negative charge that is compensated by a counterion. When the counterion is a proton, a Bronsted acid site is created. Moreover, framework oxygen atoms can give rise to weak Lewis base activity. Noble metal ions can be introduced by ion exchanging the cations after synthesis. Incorporation of metals like Ti, V, Fe, and Cr in the framework can provide the zeolite with activity for redox reactions. 

To overcome the limitations of natural zeolites a whole range of synthetic zeolites have been manufactured since the 1950s. These have tailored pore sizes and tuned acidities, as well as often incorporating other metal species. The basic synthesis involves mixing a source of silica, usually sodium silicate or colloidal Si02, with a source of alumina, often sodium aluminate, and a base such as sodium hydroxide. The mixture is heated at temperatures up to 200 °C under autogenous pressure for a period of a few days to a few weeks to allow crystallization of the zeolite. 

In order to prepare ZSM-5 zeolite nanocrystals, an A1 source of aluminium isopropoxide was added into solution A, and hydrothermal synthesis of the solution A containing Si and A1 sources was carried out in an 0-15/cyclohexane solution at 120 degree C for 50 h. Figures 4 show ac-NHj-TPD spectra and a SEM photograph of the ZSM-5 zeolite nanocrystals. Nanocrystals with a diameter of approximately 150 nm were observed, and the NH3-TPD spectrum showed desorption of NHj above 600 K, indicating that the nanocrystals possessed strong acid sites. 

The synthesis of ethylenediamine (EDA) from ethanolamine (EA) with ammonia over acidic t3pes of zeolite catalyst was investigated. Among the zeolites tested in this study, the protonic form of mordenite catalyst that was treated with EDTA (H-EDTA-MOR) showed the highest activity and selectivity for the formation of EA at 603 K, W/F=200 g h mol, and NH3/ =50. The reaction proved to be highly selective for EA over H-EDTA-MOR, with small amounts of ethyleneimine (El) and piperazine (PA) derivatives as the side products. IR spectroscopic data provide evidence that the protonated El is the chemical intermediate for the reaction. The reaction for Uie formation of EDA from EA and ammonia required stronger acidic sites in the mordenite channels for hi er yield and selectivity. 

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