Abstract
A bentonite collected at Serra de Dentro (SD), Porto Santo Island, Portugal, and the source clay SAz-1 (Cheto, Arizona) were ion-exchanged with different cations (Al 3+, Ni 2+, Cr 3+ and Na +). Variable temperature diffuse reflectance infrared Fourier transform spectroscopy (VT-DRIFTS) of pyridine treated samples, thermal desorption of cyclohexylamine and real time mass spectrometry of the evolved gases were used to evaluate the acidic properties of the prepared catalysts. The catalytic activity of these ion-exchanged clays was tested in the acid-catalysed conversion of limonene at 150 °C, to yield isomerization products (terpinolene, α-terpinene, γ-terpinene and isoterpinolene), disproportionation products ( p-cymene and p-menthenes) and high molecular-weight compounds. The possibility of increasing the selectivity toward p-cymene, over a catalyst with substantial Lewis acid character, was particularly envisaged. Catalysts derived from SD were significantly more active than their SAz-1 counterparts. This was mainly attributed to the greater inherent acidity, to the higher structural iron content (providing dehydrogenation activity) and to the higher nitrogen surface area of the starting SD clays. Within the M n+ -SD series, the order of activity decreased as Ni 2+ > Al 3+ > Cr 3+ > Na +. A direct comparison between Al 3+-SD (with maximized Brönsted acidity) and Ni 2+-SD (with predominant Lewis acid character) provided no support for enhanced p-cymene production over Lewis acid sites.
Highlights
The use of renewable feedstocks from natural sources is a growing field in which heterogeneous catalysis can offer considerable improvement
The results showed that increasing the severity of the acid treatment produced more active catalysts, but the increased catalytic behaviour did not correlate directly with either the calculated surface areas or with the estimated acidity values
It has been shown [24] that peaks at 1635 and 1540 cmÀ1 can be assigned to the pyridinium ion (Bronsted acidity, bound pyridine (BPYR)), whereas peaks at 1613 and 1450 cmÀ1 are diagnostic for pyridine co-ordinately bound to Lewis acid sites (LPYR) while peaks at 1596 and 1440 cmÀ1 may report the presence of H-bonded pyridine (HPYR)
Summary
The use of renewable feedstocks from natural sources is a growing field in which heterogeneous catalysis can offer considerable improvement. The conversion of cheaply available limonene to the more commercially valuable p-cymene (for use in fine and speciality chemicals) is a good example, and an extensive review on this topic has recently been published [1]. This conversion requires the migration of the exocyclic double bound into the ring followed by a further ring dehydrogenation. The presence of acid centres on the catalyst surface is essential in order to isomerize the double bonds in the cyclohexene ring of limonene. For this process Pd supported on a lowacidity silica support proved to be the most appropriate catalyst [5,6]
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