On the role of acidity in amorphous silica-alumina based catalysts

Abstract

Amorphous silica-alumina (ASA) is widely used as a solid acid catalyst or as a carrier for well-dispersed metal sulfide or metal catalysts. They are often involved in hydrocracking catalyst formulations, especially so when the aim is to produce middle distillates from heavy oil fractions. With increasing demand for diesel and kerosene balanced acidity in these catalysts to combine high conversion with high middle distillates selectivity is crucial. An important advantage of amorphous silica-alumina as an acidic catalyst is its open texture with substantial mesoporosity as compared to zeolites that typically suffer from diffusion limitations when bulky hydrocarbons need to be converted. Strongly acidic supports also cause excessive coke formation and overcracking of the feedstock resulting in lower middle distillate yields. Therefore, the use of ASA as the acidic component in bifunctional hydrocracking has become important. Although moderate acidity is important for ASA supports, accurate control of the acidity is hampered by a lack of understanding about the origin of acid sites in these materials and how they are formed. Regarding the former, the nature of the Bronsted acid sites (BAS) has not been unequivocally established. The more widely shared opinion is that the Bronsted acidity derives from tetrahedral Al3+ in the silica network, as initially proposed by Thomas and Tamele in the late 1940s. This proposal however has remained inconclusive. Alternative explanations for the acidity have been also been proposed. These include Lewis acidic Al ions substituting for protons of surface silanol groups and the higher acidity of silanol groups in the presence of neighbouring aluminium surface atoms The other reason that surface acidity of ASAs is understood to a much lesser extent than that of zeolites relates to the complex surface composition of these mixed oxides. ASAs are made by co-precipitation, co-gelation or grafting processes and in nearly all cases, the resulting materials contain a non-random distribution of aluminium in silica. The present project was thus undertaken with the aim of (i) synthesizing a set of ASA materials by as controlled a method as possible for use in catalytic activity studies and (ii) to learn about the genesis of Bronsted acid sites in ASAs and their strength. The synthesis method chosen was a well-defined variant of grafting, viz., homogeneous deposition-precipitation. The entire process of the deposition of aluminium on silica and subsequent calcination was followed by 27Al NMR spectroscopy . The study showed that the aluminium species in the dried precursors is a function of pH and starting aluminium concentration. At pH of 3 and at low aluminium concentration, the surface mainly consists of tetrahedral and octahedral aluminium species. Under these conditions an increase in pH gives mainly rise to tetrahedral aluminium species on the surface. This is attributed to the further condensation reaction occurring with the surface silanol groups. However, with increasing aluminium concentration, the deposition mechanism involves reaction of aluminium species in solution with species already grafted on the surface. This results in the formation of polymeric aluminium species. In addition, at higher aluminium concentrations some precipitation of aluminium hydroxide also occurs. When the dried precursors are then calcined, redistribution of the grafted aluminium species occurs, mainly with a small fraction of aluminium diffusing into the silica matrix thereby resulting in Bronsted acid sites. The formation of Bronsted acid sites upon calcination was also evidenced by n-alkane hydroconversion activity tests, which requires the presence of strong acid sites. From this systematic study the surface of amorphous silica- alumina could be described as consisting of three different species, namely a pure silica-alumina phase that originates from isolated aluminium grafted onto the silica surface, domains of aluminium oxide and a small fraction of aluminium in the silica network responsible for the strong Bronsted acidity. By following the selective H/D exchange of acidic hydroxyl groups in aluminosilicates by IR spectroscopy clear evidence was provided for the existence in ASAs of BAS comparable in strength to the bridging hydroxyl groups in zeolites. The method is able to distinguish various types and strengths of strong BAS in luminosilicates (zeolites, clays, ASAs) such as enhanced acidic sites in steam calcined faujasite zeolites. By carrying out the H/D exchange under conditions under which zeolites selectively exchange their bridging hydroxyl groups, weak bands were observed at 2630 and 2683 cm-1 in the deuteroxyl region of ASAs. By quantification it follows that the concentration of strong BAS in ASAs is 2-3 orders of magnitude lower than in zeolites. A number of techniques (CO IR, pyridine IR, alkylamine TPD, Cs+ and Cu(EDA)22+ exchange, 1H NMR and m-xylene isomerization) was used to validate the H/D exchange FTIR results and provide further insight into the heterogeneous surface composition of ASA. The results show that the surface contains both Bronsted and Lewis acid sites of varying acidity. The number of strong Bronsted acid sites of zeolitic strength is very low (<10 µmol/g). Careful interpretation of IR spectra of adsorbed CO and pyridine confirmed that the surface contains only very few of such sites. Other methods suitable to estimate strong BAS involve an adaptation of the well-known decomposition of alkylamines and titration of such sites by a base during m-xylene isomerization. These sites originate from Al substitutions in the silica network. Besides, the surface contains between 50-150 µmol/g of a weaker form of BAS, which can be easily quantified by CO IR. Cu(EDA)22+ exchange also appears to probe weak BAS. The structure of these sites remains unresolved, but some of the results suggest that these are related to paired sites, involving the interaction of strong Lewis acid sites with silanol groups. In addition, the surface of ASA contains two forms of Lewis acid sites: (i) a weaker form associated with segregated alumina domains and probably containing five-coordinated Al species that make up the interface between such domains and the ASA phase and (ii) a stronger form which are lower coordinated Al sites grafted to the silica surface.