26 February 2016
Valerii Bukhtiyarov, director of Boreskov Institute of Catalysis (Novosibirsk), reported on the prospects of using the renewable vegetable stock in the fuel and petrochemical complex of Tatarstan
The interest in Cu-ZSM-5 catalysts, which are highly active to direct decomposition of NO and selective catalytic reduction of NO with hydrocarbons including propane, gives an impetus to studies of the electron states of copper in the catalyst (oxidation level and coordination) and their influence on the catalytic properties. It is shown in a number of papers that the catalytic activity of Cu-ZSM-5 expressed as conversion of NO increases with the level of Cu-exchange to reach maximum at the ratio Cu/Al"100% [1-3]. At the further increase in Cu/Al (to as high ratio as 450% [1, 3, 4]) the NO conversion keeps constant.
The present work deals with EPR and ESDR studies of electron states of copper depending on the method used for preparation of Cu-ZSM-5 (ion exchange, wet impregnation, deposition), basic conditions of synthesis (pH of copper solution, copper precursor, temperature etc.) and copper loading.
For calcined Cu-ZSM-5 samples treated in vacuum at 400°C, axial EPR spectra of O - ion radicals with g=2.05 and g=2.02, which are presumably assigned to linear -O--Cu+-O--Cu+-O- chains in zeolite channels , are observed against the background of EPR signal of isolated octahedral Cu2+ ions with different extents of tetragonal distortion.
The studies of the copper electron states in Cu-ZSM-5 have allowed a copper state with absorption bands at the unexpected region of 18000 to 23000 cm-1 to be discovered for the first time in the samples treated in vacuum at 150 - 400°C (Fig.). Along with this state, isolated Cu2+ ions (a.b. 12500 - 14000 cm-1) and clustered copper oxide species (CTB 27000 - 32000 cm-1) are observed (Fig.). The discovered absorption bands (18000 - 23000 cm-1) relate to the ligand-metal CTB in -O2--Cu2+-O2--Cu2+-O2--like chains in the zeolite channels. Probably, the chain structures are easy to reduce (they are even capable of self-reduction) and may play an important role, along with Cu2+...Cu1+ sites (a.b. 15000 - 17000 cm-1), in selective reduction of nitrogen oxides.
ADF quantum chemical calculations argue for the probable occurrence of charge transfer bands at 18000 - 23000 cm-1 for copper with coordination number equal to 2, such as linear Cu2+-O2--Cu2+-O2-- chains, and, in principle, for the possibility for intrachain reduction Cu2+O2 --> Cu+O-.
Formation of the chains is caused by copper hydrolysis during synthesis of Cu-ZSM-5 followed by stabilization of polynuclear [Cu2+O2-] species in the zeolite channels. The ratio of copper states in Cu-ZSM-catalysts depends both on the preparation procedure and conditions (pH, concentration of copper salt solute, temperature) and on conditions of the thermal post-treatment in vacuum (temperature and time).
ESDR spectra of Cu-ZSM-5 sample, (1) - initial and treated in vacuum at 25oC (2) and 150oC (3)
Dehydro-oligomerization of methane to benzene over Mo/HZSM-5 catalysts is promising method for the catalytic transformation of methane.
Catalyst samples are prepared by impregnation of H-ZSM-5 zeolites by ammonium paramolybdate (NH4)6Mo7O24
Three types of zeolites with different module (Si/Al, 17, 34, 73) were used.
Study of Mo state in ammonia paramolybdate solutions at different
concentrations and pH (the numbers indicate pH of solutions)
Rate of C6H6 formation 73.6 nmol/g*s