Synthesis and characterization of HPW/MCM-41 (Si) and HPW/MCM-41 (Si/Al) catalysts: Activity for toluene alkylation with 1-dodecene

Solids of the MCM-41 type (BET specific surface area around 600 m²/g), with only Si and with Si/Al (atomic ratio) = 25, were synthesized and employed as supports of dodecatungstophosphoric acid (HPW)-based catalysts. Catalysts were prepared by impregnation of HPW, in amounts of 20, 40 and 60 wt%. Characterization by DTA, TGA, XRD, FT-IR and NH₃-TPD suggest an interaction of HPW with the supports. This interaction in the HPW/MCM-41 (Si) catalysts leads to a higher thermal stability of the Keggin structure of HPW and thus a preservation of its acidic protons. This effect is reflected in a higher density of acid sites upon increasing HPW loading. Powder XRD patterns of catalysts and the steady loss of specific surface area in both supports are consistent with a uniform distribution of the supported HPW. Analysis by FT-IR of the supported catalysts shows a displacement of the W-O-W band of supported HPW as compared with pure HPW. This displacement decreases with increasing loading, being higher for 20% HPW/(Si/Al). The catalytic activity in the alkylation of toluene with 1-dodecene increases in the following sequence: 20% HPW/(Si/Al) < 40% HPW/(Si/Al) < 20% HPW/(Si) < 60% HPW/(Si/Al) < 40% HPW/(Si) < 60% HPW/(Si), which is directly related to the acid site density. The conversions and selectivities towards monoalkylated products for HPW/MCM-41 (Si) catalysts are higher than those for HPW/MCM-41 (Si/Al). The 60% HPW/MCM-41 (Si) catalysts presents the higher selectivity to monoalkylated products and the higher thermal stability, while for 20% HPW/MCM-41 (Si/Al) no catalytic activity was observed.