|
Abstract:
|
In this venture three distinct class of catalysts such as, pillared clays and
transition metal loaded pillared clays , porous clay heterostructures and their
transition metal loaded analogues and DTP supported on porous clay
heterostructures etc. were prepared and characterized by various physico chemical
methods. The catalytic activities of prepared catalysts were comparatively
evaluated for the industrially important alkylation, acetalization and oxidation
reactions.The general conclusions drawn from the present investigation are
Zirconium, iron - aluminium pillared clays were synthesized by ion
exchange method and zirconium-silicon porous heterostructures were
Summary and conclusions
259
prepared by intergallery template method. Transition metals were loaded
in PILCs and PCHs by wet impregnation method.
Textural and acidic properties of the clays were modified by pillaring
and post pillaring modifications.
The shift in 2θ value to lower range and increase in d (001) spacing
indicate the success of pillaring process.
Surface area, pore volume, average pore size etc. increased dramatically
as a result of pillaring process.
Porous clay heterostructures have higher surface area, pore volume,
average pore diameter and narrow pore size distribution than that of
pillared clays.
The IR spectrum of PILCs and PCHs are in accordance with literature
without much variation compared to parent montmorillonite which
indicate that basic clay structure is retained even after modification.
The silicon NMR of PCHs materials have intense peaks corresponding
to Q4 environment which indicate that mesoporous silica is incorporated
between clay layers.
Thermo gravimetric analysis showed that thermal stability is improved
after the pillaring process. PCH materials have higher thermal stability
than PILCs.
In metal loaded pillared clays, up to 5% metal species were uniformly
dispersed (with the exception of Ni) as evident from XRD and TPR
analysis.
Chapter 9
260
Impregnation of transition metals in PILCs and PCHs enhanced acidity
of catalysts as evident from TPD of ammonia and cumene cracking
reactions.
For porous clay heterostructures the acidic sites have major contribution
from weak and medium acid sites which can be related to the Bronsted
sites as evident from TPD of ammonia.
Pillared clays got more Lewis acidity than PCHs as inferred from α-
methyl styrene selectivity in cumene cracking reaction.
SEM images show that layer structure is preserved even after modification.
Worm hole like morphology is observed in TEM image of PCHs materials
In ZrSiPCHS, Zr exists as Zr 4+ and is incorporated to silica pillars in the
intergallary of clay layers as evident from XPS analysis.
In copper loaded zirconium pillared clays, copper exists as isolated species
with +2 oxidation state at lower loading. At higher loading, Cu exists as
clusters as evident from reduction peak at higher temperatures in TPR.
In vanadium incorporated PILCs and PCHs, vanadium exist as isolated
V5+ in tetrahedral coordination which is confirmed from TPR and UVVis
DRS analysis.
In cobalt loaded PCHs, cobalt exists as CoO with 2+ oxidation state as
confirmed from XPS.
Cerium incorporated iron aluminium pillared clay was found to be the
best catalyst for the hydroxylation of phenol in aqueous media due to
the additional surface area provided by ceria mesopores and its redox
properties.
Summary and conclusions
261
Cobalt loaded zirconium porous clay heterostructures were found to be
promising catalyst for the tertiary butylation of phenol due to higher
surface area and acidic properties.
Copper loaded pillared clays were found to be good catalyst for the
direct hydroxylation of benzene to phenol.
Vanadium loaded PCHs catalysts were found to be efficient catalysts for
oxidation of benzyl alcohol.
DTP was firmly fixed on the mesoporous channels of PCHs by Direct
method and functionalization method.
DTP supported PCHs catalyst were found to be good catalyst for
acetalization of cyclohexanone with more than 90% conversion. |