Hydrothermal stability of SBA-15 and related ordered mesoporous silicas with plugged pores

Abstract

Ordered mesoporous silicas, including SBA-15 and related plugged materials, were synthesized under acidic conditions using Pluronic P123 poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) triblock copolymer (EO₂₀PO₇₀EO₂₀) as a supramolecular template and tetraethyl orthosilicate (TEOS) as a silica precursor. The TEOS : copolymer molar ratio was either 60 ∶ 1 (as in the typical SBA-15 synthesis), or it was doubled or tripled. The larger relative amounts of silica precursor resulted in the formation of constrictions or plugs in the mesopore channels, as inferred from N₂ and Ar adsorption at 77 K. As the amount of TEOS relative to the copolymer increased, the specific surface area was slightly reduced, whereas the total pore volume decreased considerably (by up to 45%), and the mesopore diameter decreased from 10.3 to 6.6 nm with retention of a narrow pore size distribution (PSD). The adjustment of the synthesis conditions allowed us to prepare a sample whose primary mesopores featured constrictions with openings of diameter about a half of the primary mesopore diameter, as determined via the surface modification with organosilanes. The calcined samples were heated in water at 373 K for periods of time from 6 hours to 8 days in order to compare the hydrothermal stabilities of SBA-15 and the plugged silicas. In all cases, the treatment in water led to a gradual increase in the primary mesopore size. A slight broadening of PSD was noticed after one day, and the mesopore size uniformity was further diminished for longer treatment times. For plugged samples prepared using a standard procedure with an excess of TEOS, the plugs persisted during the hydrothermal treatment. However, in the case of a sample prepared at lower temperature and for a shorter time, the plugs were nearly completely eliminated after 4 days of boiling, which suggests that the stability of the plugs depends on the synthesis conditions. This finding may be important from the point of view of prospective applications of plugged mesoporous silicas.