(581f) Combining VAlON & VZrON to VZrAlON Oxynitrides: A Novel Efficient Catalyst Class for the Ammoxidation of Heteroaromatics

Introduction

The
gas-phase ammoxidation of heteroaromatics
is an ecologically and economically efficient route to synthesize a variety of
different nitriles (Eq. 1), as long as high nitrile selectivities
and space-time yields (STY) are reached.

R-CH₃
+ NH₃ + 3/2 O₂à R-CN
+ 3 H₂O (Eq. 1)

A
novel approach to improve STY at a high selectivity for the ammoxidation
was initiated by the discovery of the VAlON catalysts,
providing high STY for propane ammoxidation [1].
Recently, we have found that VAlON and VZrON catalysts show promising catalytic performances in
the ammoxidation of 3-picoline (3-PIC) to
3-cyanopyridine (3-CP) [2], being an important fine chemical and used as a precursor
for the vitamin B₃ production. STY could be almost triplicated with VZrON catalysts compared to a common used benchmark
catalyst, based on mixed metal oxides [2, 3]. However, both oxynitrides
suffer from a lower selectivity compared to the benchmark catalyst. Additionally,
VZrON catalysts were proved to be highly active, but
poorly selective compared to VAlON. By ex situ
analysis (e.g. XPS, XRD) and in situ EPR spectroscopy it was found that those
differences in catalytic performances might be due to a higher VO_x site polymerisation, a higher mean V surface
oxidation state and an almost N-free surface for the VZrON,
but not for VAlON catalysts [2]. This
structure-reactivity relationship inspired us to combine the active VZrON and selective VAlON binary oxynitrides to a novel ternary VZrAlON
catalyst class. Besides the catalytic performances, the structure of VZrAlON catalysts has been characterized as a function of
the V/(Al+Zr) ratio at a constant Al/Zr ratio of 1.5, using several analytical ex situ
techniques (XRD, XPS, ATR, UV-Vis-DRS) and in situ EPR spectroscopy as well.

Results and Conclusions

As
it can be seen from Fig. 1, an improved selectivity S_3-CP is achieved
with the VZrAlON-0.5 catalyst (V/(Al+Zr)
= 0.5) at almost equal conversion compared to the VZrON-0.25 (V/Zr = 0.25) catalyst (GHSV = 5728 h^-1, T = 360
and 382 ^oC). In comparison to VAlON-0.5,
the latter VZrAlON catalyst is more active and slightly
less selective (GHSV = 2713 h^-1, T = 360 ^oC).

Also
the structural properties of VZrAlON differ
significantly from those of VAlON and VZrON. From XPS studies it was found, that the combination
of VAlON and VZrON to VZrAlON leads to a higher amount of different N surface sites
(NH₄⁺, NH₂^-, NH^2- N^3-
) and a mean V surface oxidation state close to +4. In contrast, the surface of
VZrON catalyst was almost free of any N site, while
for VAlON catalysts only nitride-like sites could be
detected by XPS studies. In situ EPR studies, performed over VAlON, VZrON and VZrAlON catalysts during ammoxidation,
suggest that only VZrAlON catalysts provide two
different types of isolated VO_x sites and
a small fraction of anionic vacancies, besides a certain amount of polymerised VO_xsites under working state. As
proved from nitridation of the VZrAlO
oxide precursor, monitored by in situ EPR spectroscopy, those anionic vacancies
are introduced via nitridation. For comparison, the
concentration of isolated VO²⁺ sites in VZrON
catalysts is significantly lower under reaction conditions, while for VAlON catalysts the shape of EPR signals point to a high
amount of isolated VO²⁺ sites. Those trends are also evident from
UV-Vis-DRS investigations, performed ex situ on the samples.

Fig. 1: Catalytic performance (X_3-PIC: conversion
of 3-PIC, S_3-CP: selectivity to 3-CP) at different reaction
temperatures T and different space velocities GHSV for the VZrON-0.25,
VZrAlON-0.5 and VAlON-0.5 catalysts (numbers denotes the V/Zr,
V/(Zr+Al) and V/Al ratios in
these samples).

Thus,
the improved catalytic performance of VZrAlON
compared to VAlON and VZrON
might be due to a fractional incorporation of Zr into
the VAlON network, suppressing the high VO_x site polymerisation, as it is the case for VZrON catalysts. In addition, the higher nitrile selectivity
might be due to the ability of the VZrAlON catalyst
to incorporate NH_x(x = 1, 2) sites
from the gas phase onto its surface and probably due to a better surface electron
transport enabled by a small fraction of anionic vacancies.

[1] Florea M., Prada-Silvy,
R., and Grange P. (2003), Catalysis Letters, 87 (1), pp. 63-66.

[2] Janke, C., Radnik
J., Bentrup U., Martin A., and Brueckner
A. (2009), ChemCatChem, 1(4), pp. 485-491.

[3] V. Hippel, L., Neher, A., and Arntz D. (1996), EP 0726092B1.