(337e) Simple Characterization of Solid Acid Catalysts By Reactive Gas Chromatography

One of the more common methods for characterizing
solid acid catalysts is the temperature programmed desorption (TPD) of
alkylamines, which selectively decompose to an alkene and ammonia over Brønsted
acid sites. TPD is traditionally carried out either in a flow reactor or
microbalance, coupled with a mass spectrometer for identification and/or
quantitative purposes. Reactive gas chromatography is a new technique to
measure Brønsted acid site densities of solid acid catalysts, which condenses
the typical experimental setup required for a TPD into a single, fully
automated gas chromatograph (GC). Reactions are carried out in a microcatalytic
reactor, housed within a temperature controlled GC inlet liner, products are
then separated in a GC column and quantified by a flame ionization detector
(Figure 1a). This eliminates the need for more complex instrumentation such as on-line
mass spectrometers or a flow reactor apparatus.

An investigation of the RGC method was carried out by characterizing
ZSM-5, BEA and SPP zeolites of varying Si/Al ratios, utilizing alkylamine
decomposition to selectively count Brønsted acid sites. A comparison between RGC
measurements and conventional TPD methods show excellent agreement (Figure 1b),
confirmed by comparison with pyridine in-situ titrations under Brønsted
catalyzed reaction conditions. Reactive gas chromatography measurements
performed using n-propylamine, isopropylamine and tert-butylamine were found to
yield identical Brønsted acid site densities. Given the relatively low
detection limits of a flame ionization detector, the method of reactive gas
chromatography is found to be highly sensitive. Siliceous materials with
Brønsted acid site densities of ~1 μmol g_cat^-1 could
be reliably measured.

Figure
1. Method of Reactive Gas Chromatography for Solid Acid Titration. A. Solid acid samples loaded into the
inlet liner (red rectangle) of a gas chromatograph are titrated with amines,
introduced via an automatic liquid sampler (ALS) syringe, adsorbed and reacted
on solid acid sites to produce propylene and ammonia. Resultant products are
quantified by a flame ionization detection (FID). B. Comparison of
zeolite measured Brønsted acid site density between reactive gas chromatography
and literature values.  Catalysts include ZSM-5 (Si/Al 11.5, 40, 140: ●)
and BEA (Si/Al 12.5, 19:  ■).