The reaction parameters (i. e. steam temperature, reaction time and particle size) which have been previously highlighted to influence the auto-hydrolytic process similarly have been demonstrated to affect the catalysed steam pre-treatment method [21, 12]. In addition to those factors, the selection of the type and concentration of acid catalyst employed, as well as the biomass type and its moisture content are also major considerations which must be addressed with the use of this method.

Although a variety of acid catalysts prior to steam pre-treatment have been demon­strated in the literature (i. e. the use of aqueous phosphoric acid (H3PO4) for the pre-treatment of sugarcane baggase [34]), the use of H2SO4 and SO2 as the process acid catalysts currently predominates acid catalysts investigated and in practice. This section will, thus, concentrate on the use of these acid catalysts. A study carried out in [35] compared the use and influence of the choice of H2SO4 (0-3 %, w/w) and SO2 (1 %, w/w) for the pre-treatment of Salix caprea (Willow) wood chips with a steam temperature range of 160-230 °C investigated with a fixed reaction time of 10 min (with a 15 min treatment of the biomass samples with saturated steam at 1 bar prior to the acid catalyst addition). With an increase in the H2SO4 acid concentrations (and steam temperatures), a reduction in the fibrous material yields were observed. This was attributable to the improved solubilisation of the extractives and hemicelluloses under these conditions [35]. The xylose yields (based on the calculated biomass xy­lan content) were also seen to increase with an increase in the H2SO4 concentrations. With a steam temperature of 190 °C and using the highest study H2SO4 concentration of 3 %, a xylose yield of 80 % was obtained, the glucose yields (after enzymatic hydrolysis) were however seen to be reduced under these conditions [35]. This was considerably higher than the <15 % xylose yields observed when no acid catalysts were used for the steam pre-treatment [35]. With the use of SO2 as the process cat­alyst, a maximum xylose yield of 62 % was observed using a steam temperature of 200 °C [35]. Correspondingly, a glucose yield of 95 % (on the basis of the of the biomass glucan availability) was obtained after enzymatic hydrolysis with the use of the same reaction parameters [35]. With glucans being the main constituents of willow biomass and with higher glucose yields upon hydrolysis considered to be more beneficial than xylose yields in the water soluble fractions, the results of that study led to the conclusion that the SO2 acid catalysts were preferable for use as the acid catalyst candidate for use in the catalysed steam pre-treatment method.

The use of SO2 concentrations (1-4 %, w/w) as the process catalyst for the impregnation of wood prior to the steam pre-treatment was seen to significantly reduce the overall process temperature and time requirements to achieve optimum biomass solubilisation, recovery and hydrolysis of the post-treated substrates [13].

The use of SO2 as the process catalyst over H2SO4 has also been discussed to be preferred in industrial catalysed steam pre-treatment systems since the former was reported to require a comparatively less expensive reactor materials, leads to the generation of less gypsum as a process by-product, and less process steam re­quirements than the former [13, 36]. The ease in which SO2 is incorporated evenly within lignocellulosic materials has also been a factor favouring its use over H2SO4, especially with regards to the handling inconveniences encountered with the soaking process necessary with the use of aqueous H2SO4 [13].

Regarding the influence of the type of lignocellulosic biomass employed in the catalysed steam pre-treatment process, softwoods have been demonstrated to require more harsh pre-treatment conditions for the production of suitable substrates aimed at effecting high product yields upon hydrolysis [23]. Based on this, the use of SO2 as an acid catalyst was shown to be beneficial when used for the pre-treatment of hardwood, but highly essential for use for the impregnation of softwood, that is, spruce and fir prior to their steam pre-treatment [23, 31]. Furthermore, with an increasing moisture content of the wood chips, it was observed that a higher efficiency of the acid catalyst (expressed in relation to the dry weight of the wood chips) was retained within the chips, subsequently resulting in an enhanced biomass pre-treatment [23].