Catalysis of Organic..

Bianchi, Borzatta, Poluzzi and Vaccari 341Yield (%)1008060402002,0 2,8Concentration of DMED in MeOH (M)Figure 4. Yield in TMBD as a function of the feed concentration. ((9 g ofMega Dry clay; T = 290 °C; P = 0.1 MPa; LHSV = 0.92 h -1 ; Feed =DMED/CH 3 OH solution in N 2 flow.saponites (Bentonites de Cabanas and Esmectita EMV 351), which showed thesignificantly lowest catalytic performances. Saponite is also a 2:1 type clay, but itis characterised by the isomorphous substitution of Si by Al atoms in thetetrahedral layers and therefore has many and strong Lewis acid sites (24, 25).In a following step, we attempted to optimise the reaction conditions, usingMega Dry clay as catalyst: the first increase in temperature (290 °C) produced anapproximate 15% increase in the yield of TMBD, while a further increase to 310°C worsened the catalytic performances. The rate of the liquid feed also played asignificant role, with an increase in the yield of TMBD up to a LHSV value ofabout 0.92 h -1 , whereas for higher values the catalyst surface was partially stuffed(Fig. 3). Lastly, the DMED concentration also played an important role, sincewhen the DMED concentration was decreased, the yield in TMBD almost halved(Fig. 4). However, according to literature (12), operating at high feedconcentration is not simple: in fact special reactor arrangements, with heated feed,are required because of the limited solubility of DMED in CH 3 OH.The screening tests in optimised conditions confirmed the excellentperformances of raw attapulgite/montmorillonite mixtures (Granosil 750 JF andMega Dry) (Fig. 5). It is worth noting that these latter catalysts showed better