Desorption of carbonium ions from zeolite surfaces; A comparison of reactive surface ionization with field ionization

Abstract

In comparison with field ionization on platinum the thermal desorption of surface carbonium ions from zeolite surfaces has been investigated mass spectrometrically. CaY-zeolites — forming carbonium ions — were supported by Pt filaments or Pt emitter tips where hydrocarbons are chemisorbed in a non-ionic form. Thermal ion desorption at very low external fields has been observed with stable carbonium ions of triphenylmethyl compounds Ph 3 C-X. This ion desorption was achieved on CaY zeolite surfaces and occurred in the order X  Br > X  Cl > X  OH; however, no ions could be obtained with X  H and X  COOH up to 825°K. On Pt surfaces high fields were required for desorbing molecular ions. The ion desorption from the zeolite and the field desorption from platinum could be distinguished by an energy analysis of desorbed ions. Those ions originating from the dielectric grains of CaY-zeolite suffer field dependent energy losses and therefore a virtual shift in the mass scale. Thermal desorption (a) and field desorption (b) differ in principle: (1) in the kind of ions formed, (a) carbonium ions from zeolite surfaces, (b) parent molecular ions from platinum; (2) in the temperature dependence of ion currents, in (a) a positive temperature function with necessary minimum temperature, in (b) no minimum temperature, and negative temperature coefficients have been observed; (3) in the field dependence of ion currents (a) very low fields < 10 5 V cm were sufficient, and Schottky-line dependences of intensities have been found, in (b) a minimum field > 10 7 V cm was required. Ion formation on zeolite surfaces is facilitated by an intra-molecular substitution of Ph 3CX, which polarizes the C δ+−X δ- bond, and as well by an intermolecular interaction of electron acceptors, for instance by [(CN) 2-C  C-(CN) 2]. Experimental results are discussed with respect of surface fields on zeolite structures, Brönsted- or Lewis acid sites, and the formation of charge-transfer complexes. In conclusion, acidic surface sites are most likely responsible for the carbonium ion desorption in this investigation.