Designing MFI-based catalysts with improved catalyst life for C3=andC5

Abstract

Light olefin oligomerization is an important alternative for the production of clean liquid automotive fuels, and can be performed in the presence of solid acid catalysts. Among them, medium pore zeolites, and especially ZSM-5, have been widely described in the open literature. In this work the relative importance of intracrystalline diffusion path lengths for the product molecules (depending on the zeolite crystal size and presence of mesopores in the crystallites) and Brønsted acid site density are discussed for two different olefins, propene and 1-pentene. Thus, ZSM-5 samples with a) the same crystallite size and different acid site density, b) the same density of acid sites and different crystallite size, c) post-synthesis generation of mesopores by different desilication severity, and d) samples with similar crystal size, mesoporosity and acid site density, but with a different ratio of external to internal acid sites, have been prepared and studied for oligomerization of propene and 1-pentene. The results obtained suggest that the properties required for a best performing catalyst (maximum conversion and lowest deactivation rate) are different for these two alkenes. Whereas Brønsted acid site density is determinant for propene oligomerization when intracrystalline diffusion path lengths are below a certain critical value, the presence of a high number of Brønsted acid sites is not sufficient in the case of 1-pentene, and additional mesoporosity becomes crucial. Thus, mesoporous ZSM-5 samples prepared by post-synthesis desilication treatments present a larger improvement in initial conversion and catalyst life for 1-pentene oligomerization than for conversion of propene.

Improved THETA-1 for Light Olefins Oligomerization to Diesel: Influence of Textural and Acidic Properties

“…At this point it is important to remark that the formation of products in the diesel range (C 10 -C 24 ) when oligomerizing propene implies at least four consecutive propene additions to take place, and if the formation of mesopores goes beyond a certain extent (mesopore volumes around 0.1 cm 3 /g), the diffusion of the primary products (belonging to the naphtha fraction) will be favoured, before being consumed in consecutive oligomerization steps to heavier diesel range products. Similar results were obtained in a previous study based on modified (mesoporous) ZSM-5 catalysts for olefin oligomerization [12].…”

“…In our case, besides reducing the number of preparation steps, the dealumination of the zeolite framework that takes place from ion-exchange followed by calcination is avoided. This was already proven to be critical for ZSM-5 based oligomerization catalysts [12] and will be shown to be even more important with Theta-1, especially for samples with a lower Si/Al ratio.…”

“…In a recent publication [12] we have shown that the combination of basic desilication treatments followed by selective dealumination with oxalic acid on ZSM-5 results in a largely improved activity and catalyst life for the oligomerization of light olefins. Here we show that this + + 2…”

“…In fact, framework Al-O bonds are susceptible to hydrolysis in acid media, and when an inorganic acid, such as HCl is used, the whole crystals can suffer acid dealumination, and this can lead to a partial destruction of the structure, as confirmed by the results enclosed in table 1. However, the use of bulky organic acids, such as oxalic acid, is known to limit the acid attack to the external surface of the zeolite crystals [12,[34][35]. Thus, besides removing the possible extraframework species present, oxalic acid will selectively remove the most externalaccessible-framework Al from the crystals without destroying the structure.…”

“…For the production of diesel, where branching of the hydrocarbons has to be limited, SSPA is, however, not suitable, and zeolite based catalysts have been proposed for gas phase [1,[9][10][11][12] and liquid phase operation [13][14][15][16]. When oligomerization of light olefins is carried out in the presence of heterogeneous acid catalysts, the reaction mechanism involves carbenium ions and the branching degree of product oligomers -isoolefins-is known to be proportional to the size of the catalysts pore.…”