Gas phase dehydration of C-6 alcohols promoted by Y zeolite and supported nafion catalysts

The gas phase dehydration of hexan-1-ol, hexan-2-ol and cyclohexanol (473 K less than or equal to T less than or equal to 573 K) has been studied over NaY, HY and Ni/NaY zeolites and the action of each has been compared with a commercially available 13% w/w Nafion/SiO2 composite, The Ni exchanged ze olite and Nafion catalysts exhibited a high selectivity in terms of all;ene formation. Condensation to generate the respective ether represented a sec ondary process that was not promoted to the same extent. Reaction over the Nafion sample was characterised by an initial increase in conversion with t ime that can be linked to a gradual swelling of the polymeric matrix by wat er produced in the dehydration step. Reduction of the Ni/NaY zeolites in fl owing hydrogen generated surface Bronsted acidity that has been characteris ed by IR, NH3 chemisorption and iodometric titration. Dehydration activity in the case of NaY and Ni/NaY was directly proportional to acid site concen tration and there was little dependence on acid site strength. The effect o f reaction time, temperature and gas space velocity on activity/selectivity is presented and discussed. Catalysis over the zeolites was accompanied by appreciable deactivation due to coke formation that was also dependent on surface acid site concentration. A high carbon (up to 50% w/w) content was recorded for the spent zeolites but the presence of nickel metal served to limit the deposition of highly amorphous carbonaceous species. The applicab ility of a reductive regeneration of the used zeolites is considered, Carbo n monoxide chemisorption/desorption and TEM/SEM analysis have revealed weak interaction(s) between the Ni crystallites and the aluminosilicate support . An appreciable growth of the supported nickel particles during catalysis was detected and is quantified. (C) 2001 Elsevier Science B.V. All rights r eserved.