Protonic zeolites formation

The catalysis by protonated zeolites, used in industrial cracking, isomerization and alkylation of hydrocarbons, involves proton transfer and formation of carbenium or car-bonium ions as reactive intermediates488,489. To understand the function of the zeolite, the reactions between CD4 and acidic hydrogens of OH groups of two zeolite samples have been studied recently490. 

ON THE MECHANISM OF HYDROCARBON FORMATION FROM METHANOL OVER PROTONATED ZEOLITES... 

Since it was first reported in 1976 that protonated ZSM-5 zeolites are excellent catalysts for conversion of methanol (and many other oxygenated compounds ) into hydrocarbons in the C - C q range the catalyst and the reactions have been intensely studied. Several aspects of the reactions leading to hydrocarbon formation from methanol or dimethyl ether over H-ZSM-5 or other protonated zeolites still remain unclear. In particular the first OC bond formation has been debated, and several mechanisms proposed (ref. 1). 

Acetic acid is known to undergo a vapor-phase ketonization reaction with formation of acetone on Brpnsted acids in general, and on proton-zeolites in particular. On large-pore zeolites in their proton form, the ketonization reaction is followed by acid-catalysed self-condensation amounting to mesitylene, mesityl oxide and phorone as main products [1], the chemistry being essentially identical to that in mineral acids. In H-pentasil zeolites with suitable acid site density, phorone isomerises to isophorone, which is cracked to yield 2,4-xylenol [1]. With propionic acid a similar chemistry occurs, but the formation of phenolics is severely suppressed by transition-state shape-selectivity effects... 

Guisnet, M., Magnouoc, P. and Canaff, C. (1986), Coke formation on protonic zeolites Rate and selectivity, in R. Setton (ed.). Chemical reactions in organic and inorganic constrained systems, Reidel Publishing Company, Dordrecht, pp. 131-140. 

Carbonylation of formaldehyde with carbon monoxide was also performed on various protonated zeolites such as H-ZSM-5, silicalite, H-MOR, H-Y, H-BEA, and MCM-41 (63). l,3-Dioxolan-4-one (l,3-DOX-4) is produced on Br0nsted acid sites of zeolite. H-ZSM-5, H-Y, and H-BEA zeolites with the three-dimensional channel system show the high activity for the formation of l,3-DOX-4. The reaction is carried out under batch conditions in dry methylene chloride solution, and trioxane is used as the formaldehyde source, at 40-180°C, 150-570 atm CO... 

The mechanism of formation of strong Brpnsted acidity on silica-alumina and protonic zeolites will be discussed later. It results from the association of covalent silica based structures with aluminum in different ways. In any case, in spite of the so-called acidity generation theories in the case of mixed oxides [33,129,130], it seems clear today [131] that these effects are not general phenomena. 

Fig. 1. a) Standard protonation enthalpy in secondary carbenium ion formation on H-(US)Y-zeolites with a varying Si/Al ratio, b) Effect of the average acid strength for a series of H-(US)Y zeolites experimental (symbols) versus calculated results based on the parameter values obtained in [11] (lines) for n-nonane conversion as a function of the space time at 506 K, 0.45 MPa, Hj/HC = 13.13 (Si/Al-ratios 2.6, 18, 60)... 

Because the pore dimensions in narrow pore zeolites such as ZSM-22 are of molecular order, hydrocarbon conversion on such zeolites is affected by the geometry of the pores and the hydrocarbons. Acid sites can be situated at different locations in the zeolite framework, each with their specific shape-selective effects. On ZSM-22 bridge, pore mouth and micropore acid sites occur (see Fig. 2). The shape-selective effects observed on ZSM-22 are mainly caused by conversion at the pore mouth sites. These effects are accounted for in the hydrocracking kinetics in the physisorption, protonation and transition state formation [12]. 

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. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

The formation of protonated H+(H20)n species can affect the acidity of the non-solvated protonic sites. Therefore, as the acid strength of the protonic sites in zeolites plays a key role in the hydrocarbon transformation reactions, driving the rate of the hydrocarbon protonation [4-6], the presence of water vapor among the reactants can modify reaction rates of the individual reactions involving in the hydrocarbon transformations. 

Propylene cokage experiments followed by gravimetry have shown that higher is the 5A zeolite calcium content, higher are the cokage kinetics and carbon content inside the pores (Fig. 1). The total carbon contents retained in the porosity after desorption at 350°C of physisorbed propylene are 14.5% and 11% for 5A 86 and 5A 67 samples respectively. These carbon contents are relatively important and probably come from the formation of heavy carbonaceous molecules (coke) as it has been observed by several authors [1-2], The coke formation requires acid protonic sites which seems to be present in both samples but in more important quantity for the highly Ca-exchanged one (5A 86). 

The formation of heavy carbonaceous compounds in 5A calcium exchanged zeolites depends on the calcium content. These zeolites are able to protonated ammonia molecules in ammonium ions. This Bronsted acidity results from the presence of CaOH+ species which are formed by water dissociation on Ca2+ ions and have an IR signature at 3515 cm"1. 

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