As part of a program
to investigate aspects of surface chemistry
relevant to methyl chloride synthesis catalysis, the interaction of
methanol with η-alumina doped with either CsCl or KCl in the
range 0.01–1.0 mmol g
(cat)
–1
is
investigated by a combination of diffuse reflectance infrared Fourier
transform spectroscopy and temperature-programed desorption (TPD).
Infrared spectra (IR) recorded at 293 K show that increasing the concentration
of the group 1 metal chloride progressively decreases the surface
concentration of associatively chemisorbed methanol and changes the
environment in which the adsorbed methanol resides. For CsCl concentrations
of ≥0.6 mmol g
(cat)
–1
, chemisorbed
methoxy species dominate the IR spectrum, while TPD studies show that
the amount of methanol adsorbed onto the surface, and subsequently
desorbed unchanged, changes relatively little. In the TPD experiments,
some of the adsorbed methanol reacts to give dimethyl ether (DME)
which then desorbs; for dopant concentrations of 1.0 mmol g
(cat)
–1
, DME formation is suppressed to below the limit
of detection. Unexpectedly, the presence of formate species generated
at 293 K is also observed spectroscopically, characterized by a ν
asym
(COO) mode which exhibits a hypsochromic shift relative
to potassium formate; surface concentrations of formate are higher
at higher loadings of group 1 metal chloride. Temperature-programed
IR spectroscopy shows that the room-temperature formate species desorbs,
decomposes, or migrates on warming to 653 K. Thermal ramping of the
methanol-saturated surface also results in formate production but
one that exhibits an IR profile in agreement with earlier observations
and literature values. Increasing the concentrations of the group
1 metal chloride progressively decreases the presence of the thermally
induced formate moiety. The study not only reinforces the concept
of group 1 metal chloride additives progressively rendering ineffective
those Lewis acid sites present at the η-alumina surface which
convey discrete reaction characteristics [
e.g.
, (i)
dimerization of methanol to form DME and (ii) an activated methoxy
→ formate transition] but also suggests the generation of reactive
sites not present in the undoped alumina.