AN IN-SITU DIFFUSE-REFLECTANCE FTIR STUDY OF THE CYCLODIMERIZATION OF1,3-BUTADIENE OVER CU-EXCHANGED ZEOLITES

The Diels-Alder cyclodimerization of 1,3-butadiene, to 4-vinylcyclohex ene was followed by in situ diffuse reflectance FTIR spectroscopy on a series of Cu-zeolite catalysts. The catalysts were prepared by exchan ging FAU-, BEA- and EMT-type zeolites with copper acetate. After activ ation under He, the catalysts were reduced in situ at 523 K by diluted NH3 to generate the Cu+ active sites. The reaction was then carried o ut at 413 K after stripping NH, at 473 K. The catalysts with the short est lifetimes, i.e. HFAU and 1.2CuFAU, exhibited acidic framework hydr oxyl groups with IR OH stretching Vibrations at 3640 and 3550 cm(-1), liberated after NH3 stripping at 473 K. The Bronsted acid sites cataly ze the oligomerization of 1,3-butadiene yielding oligomeric residues w ith a high CH3/CH2 ratio composed of polyolefinic species and alkenyl carbocations. On the catalysts showing little deactivation, i.e. 4.9Cu FAU and 6.9CuBEA, the carbonaceous deposits are the result of consecut ive Diels-Alder reactions on Cu+ sites, leading to polycyclic naphthen es with a low CH3/CH2 ratio. The 6.7CuEMT sample, a very active cataly st with a high content of free hydroxyl groups after NH3 stripping at 473 K, suffers from both processes of deactivation, viz. consecutive D iels-Alder reaction to polycyclic naphthenes which then transform in p art into aromatics. (C) 1998 Elsevier Science B.V. All rights reserved .