Catalysis of Organic..

110Oxidation of n-Pentanemethacrylic acid, and others as well. In some cases, this activity has led to thesuccessful development of new industrial processes.One reaction that has been investigated in recent years is the direct oxidation ofn-pentane to maleic anhydride (MA) and phthalic anhydride (PA) (2-4). The wideavailability of this alkane as a raw material is a consequence of the removal of lighterhydrocarbons from gasoline. A new process for the transformation of thishydrocarbon to valuable chemicals would be of industrial interest and might competewith current industrial technologies for the production of organic anhydrides. Thereaction starts from a C 5 alkane and yields oxygenated C 4 (MA) and C 8 (PA)compounds, in a single catalytic passage. The catalyst able to promote this complexreaction is a V/P mixed oxide, having composition (VO) 2 P 2 O 7 (vanadylpyrophosphate, VPP). The latter is also the industrial catalyst for the oxidation of n-butane to MA; it represents an example of how heterogeneous systems for theselective oxidation of alkanes must combine different types of active sites, whichcooperate in order to obtain the final desired product. The aim of the present work isto study how the surface properties of VPP affect the distribution of products in n-pentane selective oxidation.Results and DiscussionVanadium is present as V 4+ in stoichiometric VPP; however, the latter can host V 3+or V 5+ species as defects, without undergoing substantial structural changes (5,6).Therefore, the role of the different V species in the catalytic behavior of VPP in n-butane oxidation has been the object of debate for many years (7-9). Moreover, thecatalyst may contain crystalline and amorphous vanadium phosphates other than(VO) 2 P 2 O 7 (10); for instance, outer surface layers of V 5+ phosphates may develop inthe reaction environment, and play active roles in the catalytic cycle. This isparticularly true in the case of the fresh catalyst, while the “equilibrated” system(that one which has been kept under reaction conditions for 100 hours at least)contains only minor amounts of compounds with V species other than V 4+ .Much less studied has been the role of V species in n-pentane oxidation to MAand PA. Papers published in this field are aimed mainly at the determination of thereaction mechanism for the formation of PA (2-4,11). Moreover, it has beenestablished that one key factor to obtain high selectivity to PA is the degree ofcrystallinity of the VPP; amorphous catalysts are not selective to PA, and theprogressive increase of crystallinity during catalyst equilibration increases theformation of this compound at the expense of MA and carbon oxides (12). Also, theacid properties of the VPP, controlled by the addition of suitable dopants, were foundto play an important role in the formation of PA (2).In order to understand the role of V 5+ species in the formation of MA and PA,we carried out oxidizing treatments on a fully equilibrated VPP. These V speciesundergo quick transformation under reaction conditions, but their transient catalyticperformance may furnish indications on their role in the formation of products. Table