A search made in the database Current Contents Con­nect® for key words "lignocellulosic biomass" and "pre­treatment" resulted in a total of 1217 articles published from 2000 to 2012. From the total, 91.2% corresponded to research papers and 8.8% to review papers (Figure 4.1). The number of published papers has increased exponentially from 2007 to 2012 as shown in Figure 4.1, indicating the relevance that the topic has gained in the recent years. Among the reviews docu­ments, 37.2% corresponded to reviews directly related to pretreatments, pointing out the importance of this step in the concept of biorefinery. The remaining docu­ments corresponded to reviews on the general topic of biofuels from lignocellulosic biomass.

For the research papers, the search was refined to select only papers that focus on the pretreatment pro­cesses obtaining a total of 692 papers and around 54% of these papers were published in 2012. The research papers related to pretreatment were classified into nine categories as shown in Figure 4.2. From this analysis we can conclude that alkaline, acid, thermal and IL pre­treatments are the most reported, the comparison and combination of them is also widely reported. However, this variety of studied pretreatments indicates that there is not a prevalent pretreatment suggesting that further investigation on the topic is required.