DFT Study On Catalytic Dehydrogenation Mechanism Of C2-C4 Alkanes Over Pd₄ Cluster

The alkane catalytically processed to produce valuable olefinic products,which are building blocks for a wide series of commodity and specialty chemicals.However,the reaction mechanism reached by different authors is not unanimous.Reaction mechanism and kinetics for dehydrogenation of alkane,ethane,propane and n-butane,are analyzed by the B3 LYP functional with the standard split-valence 6-31G** basis set for H and C atoms and the LANL2 DZ basis set/effective core potential for Pd atoms.Palladium nanoparticle catalyst was represented by a tetrahedral cluster with single,double and triple spin multiplicity,respectively.It is found that negative Pd₄ cluster with double spin multiplicity has the best activity and selectivity.Spin multiplicity can be tuned by metal-support electronic interaction by Lewis base supports.Adsorption energy,C-H bond length,corresponding stretching vibration frequency and so on are analyzed for alkane adsorbed state.It is confirmed that C-H bond but not C-C bond in the physisorbed alkane is activated even if no covalent bond is formed.C-H bond cleavage energy barrier,prefactor and reaction rate constant of alkane are calculated by transition state theory.It is confirmed that rate-determining step is the first C-H bond cleavage with barrier of 1.21,0.96 and 1.01 eV,while rate constant is 4.04,5.81E+04and 2.00E+04s-1,respectively.Prefactor of the first C-H bond cleavage is lower by 2000,10 and 500 times than the second one,respectively,which is induced by huge entropic loss caused by alkane adsorption from gas free state.C-C bonds cleavage barrier are also calculated as 1.79,1.84 and 1.87 e V,respectively,which is such higher than C-H bond cleavage.Reaction rate constant is better descriptor than only energy barrier for the activity and selectivity for the dehydrogenation of alkane.Location of dissociated H atom will strongly influence energy barrier of the second C-H bond cleavage.There are two kinds of images for the second C-H bond cleavage,bridge site and top site,caused by location of the first dissociated H atom.For bridge site,its barrier is relatively higher than top one,and corresponding product is not suitable for two dissociated Hatoms combination and desorption.Barrier for top site of three alkanes are 0.82,0.80 and0.55 eV,while rate constant is 6.30×106,7.23×106and 2.29×10 s-1,respectively.To further study the selectivity of alkane catalyzed to alkene,the third C-H bond breaking,cleavage of alkene,is analyzed through kinetic and alkene desorption.Without hydrogen molecule co-adsorbed on cluster,rate constant for C-H bond cleavage is higher by100 times than that with H2 adsorbed for ethylene,105 times for propylene,400 times for2-butylene,while just 4 times for 1-butylene.It is indicated that co-adsorbed H2 decreases reaction rate of deep dehydrogenation and improves the selectivity to aklene.Therefore,it is proposed to introduce a little hydrogen molecule into feed gas to improve selectivity of alkane catalyzed to alkene,but not too much to decrease conversion rate.On the other hand,n-butane prefers to form 1-butylene,7.09 times as 2-butylene by kinetics analysis.Meanwhile,co-adsorbed hydrogen molecule helps to control the the distribution of n-butane to1-butylene or 2-butylene.