Cesium hydrogenophosphate

A bifunctional acid base mechanism (Figure 14.5) was proposed to explain the high selectivity to O-alkylation of rare-earth phosphates. The absence of C-alkylated products by methylation of ga iacol tends to suggest that the guaicolate surface intermediate has a different behaviour on LaP04 and LaP04 doped with cesium hydrogenophosphate. In this latter case, the excellent selectivity to O-alkylation could be due to the softness of the cesium ion (39). 

Vapor phase catalytic alkylation of phenols with methanol was carried out on various phosphates as catalysts. The best activity and selectivity was observed on boron, rare-earth and niobium phosphate. With boron phosphate, the reaction is very selective for O-alkylation even at high temperature. On this catalyst o-methoxy-phenol is selectively obtained from 1-2-dihydroxybenzene. With rare-earth phosphate calcinated at 400°C and with niobium phosphate, O-alkylation selectivity decreases with an increase of reaction temperature. For rare-earth phosphates it is possible to improve the selectivity by calcination at higher temperature or by a wetness impregnation of cesium hydrogenophosphate. An explanation of these results is proposed. 

Cesium phosphate synthesis is a little special due to its high solubility in water and cesium phosphate crystals were obtained by water evaporation from an aqueous solution of cesium hydrogenophosphate resulting from the neutralization of an aqueous solution of cesium hydroxyde by phosphoric acid. 

On the other hand, if we reported the activity as a function of specific surface area (Figure 2) we observed a very high activity of cesium hydrogenophosphate, all the other active catalysts showing a comparable activity. 

The influence of the cesium hydrogenophosphate content was examined in the case of lanthanum phosphate. Figure 1 presents the selectivity observed at 330°C for a 80 % conversion. 

This curve shows that an excellent selectivity in O-alkylated products is obtained starting at about 5 % of Cs2HP04. This value corresponds to the cesium hydrogeno-phosphate quantity which is necessary to obtain a monolayer. The increase in the cesium hydrogenophosphate content does not lead to a lower activity. 

In addition, cesium hydrogenophosphate has an interaction with the rare earth phosphate as it was demonstrated by further characterization. The mechanism in basic catalysis goes via a phenolate anion that can be chemosorbed either by an acidic site (path A) or on a neighbouring Lewis acid site (path B). Methanol will be activated by an acid site and therefore will be able to react in O or C alkylation. The C alkylation will be favoured as the acidic site chemosorbing the phenolate will be harder. For example, the excellent selectivity in O-alkylated products observed on cesium hydrogenophosphate is due to the softness of the cesium ion,... 

In this study, we have demonstrated that boron, niobium and rare earth phosphates are excellent catalysts for the selective O-alkylation of pyrocatechine in guaiacol and veratrole. The reaction is conducted in the vapor phase using methanol as the alkylating agent. In the case of rare earth phosphates the calcination temperature has a very important effect on the selectivity of the reaction. This phenomenon, linked to the synthetic procedure, is due to the residual basicity on the rare earth phosphate calcinated at 400°C. Wetness impregnation of rare earth phosphate by cesium hydrogenophosphate give rise to very active as well as selective catalysts. 

CATALYSIS BY RARE EARTH PHOSPHATE HI. CHARACTERISATION OF SAMARIUM PHOSPHATE AND SAMARIUM PHOSPHATE-CESIUM HYDROGENOPHOSPHATE AS KEY CATALYSTS FOR O-ALKYLATION OF PHENOLS... 

Samarium phosphates, impregnated or not by cesium hydrogenophosphate, selective catalysts for O-alkylation of phenols, have been characterised by various techniques. This study has shown that ... 

Particularly, a synergism between cesium hydrogenophosphate and samarium phosphate has been observed for the O-alkylation of dihydroxy-benzene (ref. 2). We described in this paper some characterizations of this solid doped or not, that may allow to explain catalytic results. 

Samarium phosphate was prepared by wet synthesis starting from samarium carbonate (Sm2(C03)3, originated RP). Precipitation of phosphate by phosphoric acid is conducted at 80°C by addition of a samarium carbonate suspension in a vessel containing phosphoric acid. After the end of the addition, the solid could be treated by ammonia at pH = 9. Cesium hydrogenophosphate is introduced by wetness impregnation of the dried (110°C) solid. 

---