348
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
Anthony J. Duben
Huckel Theory Examination of the Hydrodesulfurization of Thiophene and the Monomethylthiophenes Anthony J. Duben Division of Sclence and Engineering, Triton College, River Grove, Illinois 60 171 (Received August 29, 1977)
The Lipsch and Schuit model for the hydrodesulfurization of thiophene and the monomethylthiophenes is examined using simple Huckel theory. The optimal situation requires that the metal have its two d orbitals which are of proper symmetry to interact with the thiophene a orbitals doubly occupied with electrons. The sequence of reactions in the Lipsch and Schuit mechanism is elaborated in the light of ESCA experiments on cobalt-molybdenum catalysts. The presence of four d electrons, as mentioned, would imply that Mo3+should be identified as the effective metal species involved in the reaction on this class of catalysts. The role of cobalt is suggested to be that of a promoter for the heterolytic scission of hydrogen molecules. Comparison of the desulfurizations of thiophene and the monomethylthiophenes indicates that experimental activation energies correlate with the theoretical activation energies for the first hydrogen attack on an a carbon leading to the first C-S bond breaking. The Huckel calculations also suggest that the frequency factor in the Arrhenius rate equation is related to the energy of activated chemisorption of a thiophene which determines the relative number (apparently low) of adsorbed molecules in a configuration leading to reaction.
Introduction The hydrodesulfurization of thiophenes on the surface of a heterogeneous catalyst is a reaction whose mechanism has been the subject of extensive investigation1 because of the technological and environmental importance of the removal of sulfur from petroleum. The general conclusion appears to be that simple thiophenes experience C-S bond scission before all saturation of the ring is removed, thereby releasing a diene as a primary product. This hydrocarbon can then become more fully hydrogenated to alkenes or alkanes.
The Basic Model Lipsch and Schuit2 have proposed a reasonable physical model for the desulfurization of thiophene (see Figure 1). They surmise that the thiophene sits on the surface of the catalyst with its sulfur bound to the metal ion in the surface. The carbons a to the sulfur become subject to attack by hydrogen ions furnished by neighboring OH or SH groups. A butadiene molecule is the primary hydrocarbon product which can be further hydrogenated or desorbed as is. Simple Huckel theory was chosen to investigate the Lipsch and Schuit model in this study. Although the determination of the values of the Huckel parameters is somewhat arbitrary, it is felt that the trends observed and the conclusions drawn will also be found in other reasonable parameterizations. Parameters appropriate for the thiophene molecule were given as set B-3 in a review article by Zahradnik.3 In this parameterization a ( S ) = a(C) + P(CC), a(C,) = a(C) + O.lP(CC), and @(Cas)= O.B/3(CC). All of the other integrals are the usual Huckel theory parameters. The thiophene parameters were kept fixed throughout the calculations. A few trials were run with other parameters. The only differences noted were some small changes in orbital coefficients. Hence, it was decided that the thiophene parameters would be kept as specified in Zahradnik’s set B-3. Using these Huckel parameters, the a molecular orbitals of the unadsorbed thiophene were obtained. The energies, symmetries, and occupation of these orbitals are depicted 0022-3654/78/2082-0348$0 1.OO/O
in Figure 2. The open and solid circles represent positive and negative weightings, respectively, of the atomic orbitals a t that atom. When no circle is indicated, the coefficient is zero. The metal atom has two d orbitals suitable for bonding to the a system of thiophene. If the metal atom is considered to be at the origin of a three-dimensional Cartesian coordinate system and the thiophene is in the yz plane, the d,, can form bonds with thiophene a orbitals of bl symmetry, and the d,, can form bonds with thiophene T orbitals of a2 symmetry. The other metal d orbitals can form only 0 type bonds. (See Figure 3.) In order to reduce the bond orders of the C-S bonds in thiophene, it would seem reasonable to assume that the metal d orbitals would have to donate electrons to the antibonding a orbitals of thiophene. However, the extent of electron donation to the ring would depend on the magnitudes and signs of the Huckel parameters used to describe the metal d orbitals. First, a(d,,) = a(C) hP(CC) and P(d,,S) = kP(CC) were determined. The parameters h and k were varied until values were found which made a significant lowering of the C-S bond order (from 0.51 to 0.09). This was found to occur for h 5-1.25 and k I 0.25. The metal d,, orbital must be higher in energy than the sulfur 3p and carbon 2p orbitals. A s bond order of 0.43 arises between the sulfur and the metal d,,. Not only is the C-S bond order substantially reduced, but also the free valences and charge densities on the carbon atoms in the thiophene ring, especially the a carbons, become considerably larger. As the C-S a bond order is reduced, the a carbons should become more reactive. The inclusion of the metal d,, orbital which can also interact with the thiophene system results in further changes in bond orders and charge densities. Fixing h and h for the d,, orbital at their previously determined values of -1.25 and 0.25, respectively, will yield a C-S a bond order of 0.0 when h’(d,,) = -1.35 and k’(d,,C,) = h0.1. (The f sign on the k’ parameter is used because of the antisymmetry with respect to reflection in the xz plane of the d, orbital.) Also, the d,,S a bond order decreases from 0.43 to 0.0 when the d,, is included since the electron
+
0 1978 American Chemical Society
Hydrodesulfurization of Thiophene and the Monomethylthiophenes HC -CH 1 I
H
0
H
0
I1
Ho bHC ' s JH
0
I 4+ ! 0-M-0-M-0-M-0 I l l
t
I
C4H4S ->
l
H0 l
0-M-0-M'O'M-0 I l l
I
v
\b
o
s
0
I
l
l
2Hz -7
I I t I O-M-O--M~O-M-O
H
E
o
0
1
1
1
I I O-M-O-M~+O-M-O
+
H ~ S
t
CH,=
CH-CH=CH,
Figure 1. Reaction mechanism for the hydrodesulfurization of thiophene proposed by Lipsch and Schuit (Figure 11 of ref 2).
-
0-1.59138
a2
a -1.02568
a r2.1256
LI
e
Flgure 2. Thiophene
?r
9
molecular orbitals.
2
Figure 3. Geometrical orientation of thiophene and metal.
occupancy of the molecular orbitals of the composite system indicates that the entire manifold of bl orbitals becomes occupied. It is the completion of the bl set of the composite molecular orbitals which accounts for the large changes in C-S bond order.
The Journal of Physical Chemisfry, Vol. 82, No. 3, 1978 349
If the parameters for the metal and thiophenic orbitals are truly appropriate, then it would seem that the thiophene is not strongly bound to the metal since the only really strong bond between the sulfur and the metal would be the d,LS u bond. While the inclusion of the d,, orbital has its greatest effect on the charge densities of the metal and the sulfur and the bond orders which involve these atoms, the carbons in the thiophene ring experience changes in their charge densities and bond orders among themselves of only a few percent. For example, the largest change is in the charge density on the 01 carbons which goes from -0.67 to -0.74 when the d,, is included. The parameters proposed for the metal d orbitals are reasonable quantities. An argument in support of this claim can be made on the basis of orbital energies. If one equates the SCF energies4 of the two highest occupied T molecular orbitals of thiophene (-9.03, -9.31 eV in the fully polarized basis) with their Huckel molecular orbital energies, the values 01 = -8.38 eV and P = -0.93 eV are obtained. The energies of the d,, and d,, orbitals would be -7.22 and -7.12 eV, respectively. For comparison, the d orbitals in tetrahedral MoS2were found to have energies of -6.33 and -7.54 eV.5 Since these values bracket the previously determined d orbital energies, it may be surmised that the h values of -1.25 and -1.35 are reasonable. The choice of MoS2- as a species against which comparison was made was governed by the knowledge that molybdenum is frequently used in hydrodesulfurization catalysts in the sulfided state and by the availability of data. It is felt that the d orbital energies of sulfided molybdenum on the surface of a catalyst should not differ much from those exhibited by the metal in the tetrathiomolybdate(V1) anion. At first sight one might question the appropriateness of using a species in which the oxidation number of molybdenum is +6 for the purpose of comparing a molybdenum in the surface for which the d electron population is substantial. The population analyses for the molybdate, thiomolybdate, and selenomolybdate ions reported in ref 5 indicate a substantial d electron occupation for the molybdenum (4.6, 5.0, and 5.1 for the respective anions). Although population analyses can be changed greatly by changing basis set parameters, the possibility of finding large d orbital populations in the molybdate anions suggests that the use of the thiomolybdate anion for the purpose of making comparisons is of some utility. The increase in d orbital population on the molybdenum with the replacement of oxygen with sulfur or selenium is representative of the overall reduction of charge on the central atom when the substitution is made. There is more extensive charge transfer between sulfur (or selenium) and the metal than between oxygen and the metal. With the increase in charge transfer to the metal on sulfur substitution, there is also an increase in d orbital energy. The Mood2-d orbital energies are 7.84 and -9.52 eV, and h values corresponding to these energies would be such as to yield no C-S bond order weakening in the associated thiophene. Sulfur increases the electron density on the metal atom, making the d orbital energies sufficiently more positive so as to permit the desired reaction. This may be the reason why it is common practice to sulfide hydrotreating catalysts in the refining industry. As will be seen, charge transfer is verified by ESCA measurements on pure compounds and on commercial catalysts containing molybdenum. Commercial hydrotreating catalysts have commonly used molybdenum or tungsten, which is quite similar to
350
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
M
Anthony J. Duben
I
M
I
‘I
M
M
Figure 4. Proposed mechanism of hydrodesulfurization of a metal.
molybdenum in its chemical properties, but not chromium which acts poorly in these catalysts. This behavior may be interpreted in terms of both the h and k parameters determined for the metal d orbitals. Chromium, since it is a smaller atom with fewer electrons to shield its d electrons, will have d orbital energies substantially more negative than the d orbital energies for the large, more shielded molybdenum or tungsten. This will be reflected in the less negative h parameters used to describe the chromium d orbitals. If the h parameters are more positive than the values which had been determined, the desired weakening of the bond orders will not occur. Furthermore, it has been found that k must be small for the catalytically active metals. If k is assumed to be roughly proportional to the overlap between the orbitals in question, then the smallness of k would reflect the diffuseness of the molybdenum 4d orbitals or of the tungsten 5d orbitals. A large diffuse d orbital will have a smaller overlap with the more compact 3p of sulfur than would a chromium 3d orbital. The larger possible value of k could also prevent chromium from acting as a catalytically active metal.
The Hydrodesulfurization Reaction If the metal atom has a d4 configuration (two in the d,, and two in the dxy),then a plausible mechanism may be depicted as in Figure 4. The numbering system which will be referred to in the subsequent discussion is indicated in I in Figure 4. Also, in this discussion, energetic effects due to changes in the u bonds are not mentioned since the model used does not have the capability of giving these quantities. The formation of a bonded metal-thiophene entity (11) has a net K bonding energy of 0.66p with respect to the separated thiophene and metal. When this adduct is formed, the charge transfer between the metal and thiophene is such that the metal is now +2 with respect to its original K electron occupancy. However, the u donation from the thiophene lone pair to the d: (assumed to have been empty) will tend to reduce this charge shift. The thiophene carbons bear a net charge of -0.74 in the a position (C, and C4) and -0.26 in the /3 position (C2and C,) as compared to -0.1 for each carbon in the free thiophene molecule. The charge on sulfur is reduced from 0.41 in the free thiophene to essentially zero. The C1and C4 are now likely candidates for proton attack.
Figure 5. Plot of Mo(3d5,*) binding energy vs. calculated charge on Mo. (See ref 6 and 7.) Experimental points correspond to the following compounds: from ref 6 (1) Mo metal; (2) MoCI,; (3) MoCI,; (4) MoC15; (5) MOO,; (6) MoS,; (7) MoSe,; (8) MoOp(acac),; (9) MOO,; (IO) Na2Mo04.2H20;(1 1) (NH4)BMo7024-4H20; from ref 7-12, same as 11; ( 13) (NH,),Mo,O,; (14) CoMo04; (15) CMA-S; (16) CMA-1; ( 17) CMA-2.
In I1 the metal, as mentioned previously, is a participant in charge transfer from the metal to the thiophene for the H electrons and from the thiophene to the metal for the u electrons. The dative 0 bond will tend to increase the overall electron occupancy of the metal by roughly one electron (depending on the relative energies of the empty d,z on the metal and the sulfur lone pair). In the opposite sense, the metal atom loses two K electrons to the thiophene. Hence it can be said that the metal atom experiences a net charge loss of approximately one electron, i.e., the metal’s oxidation state increases by one. If, as Lipsch and Schuit propose, the metal atom (Mo) is in the +4 oxidation state, then adsorption of the thiophene can give the Mo an effective oxidation state of +5. This oxidation state has been suggested as being important in the hydrodesulfurization reaction; however, debate on this issue is still vigorous (see ref 1). It may be, as indicated here, the Mo5+is not the initial oxidation state of the metal, but rather an intermediate. Since Mo5+has been identified in sulfided catalysts, it may be due to structures such as VI in which HS is bound to a site yielding effectively an oxidation state of +5 on molybdenum. However, according to the scheme presented in Figure 4, this molybdenum would not be catalytically active until the HS had been stripped from the surface. It should be pointed out, nonetheless, that the formal oxidation state of the metal is not the same as its effective charge. The electron charge actually borne by the metal ion is most likely different from that suggested by the formal oxidation state which would be the charge of an isolated ion only. A series of ESCA experiments by Grim and Matienzo on a series of molybdenum compounds6 indicate a linear relationship between Mo(3dsiz) binding energy and charge using Pauling’s electronegativity scale for individual oxidation states. For sodium molybdate or ammonium heptamolybdate in which Mo has a +6 oxidation state, they find an effective charge of +1.5 on the molybdenum. Likewise, for the series MoOz, MoS2, and MoSe2 in which Mo has a formal oxidation state of +4,the metal charges are 1.2, 0.12, and approximately zero, respectively. These net charges result from the occupation of molecular orbitals rather than from the formation of ionic crystals. If the correlation plot between binding energy and charge in ref 6 is extended to include the ESCA results for
Hydrodesulfurization of Thiophene and the Monomethylthiophenes Co-Mo catalysts given by Armour et we see that in Figure 5 Mo6+supported on alumina with Co as a promoter (CMA-1,2) becomes more positive relative to the molybdenum in ammonium, sodium, or cobalt molybdate with an effective charge of approximately +2.5. The sulfided catalyst (CMA-S) has a lower charge on the Mo (approximately 2) probably due to the increased charge transfer capability of S as indicated by the MoOa, MoSz series. If in the sulfided catalyst Mo has a net charge of +2, it can have four d electrons present to interact with the thiophene. Furthermore, in the course of the mechanism presented in Figure 4,Mo as an intermediate will bear an effective charge of +3 which is reduced again to +2 when product sulfur is removed. At this point, it is interesting to note that, in the reviews of ref 1,alternative suggestions have been presented which advocate the involvement of Mo3+ in hydrodesulfurization. This theoretical study supports this latter claim based on the ESCA results and the need for four electrons on the metal in d orbitals for the best desulfurization. Support for Mo5+ is less strong since it is based on the formal oxidation state rather than effective charges which seem to be more directly related to experimental study. Further study of this question would possibly involve a careful reexamination of the assumptions used in determining the charges for ESCA on the one hand and the assignment of molybdenum oxidation states in the ESR experiments on the other.* In the proposed mechanism in Figure 4, the ring is protonated in I11 as depicted with a rather low P localization energy of 0.45p. The proton can come from a neighboring OH or SH group as suggested by Lipsch and Schuit. The acidity of this proton can be influenced by the support and the metal ion since, as the metal ion increases charge on adsorption of the thiophene, electrons will be attracted to it from and through the support. Thus, the acidity of the proton will be affected by the conductivity of the support. The S-C1 bond can break (IV) to give the pair of electrons which were in this u bond to the carbon to be included in the P system of the adsorbed butadienylmercaptan which results. Removing electrons from the sulfur produces a vacancy on this atom suitable for bonding to a hydride ion which can be produced in the heterolytic bond breaking of adsorbed hydrogen. The acquisition of a hydride ion a t this stage completes the bonding octet around the sulfur preventing an energetically favorable (by 2.568) reorganization of electrons. In this reorganized form, an increased metal-sulfur P bond order and further removal of two electrons from the metal will result. This will persist through the rest of the desulfurization mechanism leaving a sulfide on the metal at the end with a metal-sulfur bond order of 0.22 which should be more difficult to remove than the HS group in VI which has a metal-sulfur bond order of 0.0. The presence of strongly adsorbed sulfur will reduce the turnover at the catalytic site thereby diminishing the activity of the catalyst. In order to prevent the formation of persistently adsorbed sulfur, an active hydrogenation component is needed to furnish both H+ to attack the ring and H- to prevent electron reorganization. One can speculate that this may be one of the functions of the cobalt promoter in a cobalt-molybdenum catalyst in addition to its expected ability to hydrogenate butadiene and butenes. The chain species in IV can undergo a second electrophilic H+attack a t C4 to give V with a localization energy of 0.67P. Although this localization energy is higher than that in going from I1 to 111, the original ring hy-
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 351
drogenation (I1 to 111) is probably the rate-determining step since in this step resonance energy in the ring is disrupted. That this is reasonable may be inferred from considering the ease with which mercaptans are desulfurized compared to thiophenes. In forming the butadiene in VI from V, the S-C4 bond breaks heterolytically with the pair of electrons now going to the sulfur. Neutral butadiene and a H-S-metal fragment are formed. The sulfur has a complete octet, and the P electron occupancy is such so as to give a zero P bond order between the metal and sulfur, as was previously mentioned. In going from V to VI, a favorable P electron energy change of 1.64p is found. The butadiene produced can be desorbed as is, or it can proceed to be hydrogenated further to butenes and butane. In the case of a metal with two electrons in the d,, and only one in the d,, orbital, the same scheme presented in Figure 4 is appropriate. The Huckel calculations for a d3 metal interacting with a thiophene give similar changes in energy from step to step compared to the d4 metal case just discussed. The similarity suggests that the reaction can proceed on a d3 metal as well as on the d4 metal. However, certain differences are present. If hydride ions can be provided to the sulfur during the first C-S bond breaking (IV), the resulting metal-S-H fragment will have a 0.11 bond order in VI (instead of 0.0 for the d4 metal atom). This would indicate a bit more difficulty in removing the sulfur from the metal to regenerate the site. If it should happen that there are insufficient hydride ions available to act in IV, the electronically reorganized path will produce metal-sulfide fragment with the same bond order of 0.11, This fragment should be easier to break up to regenerate the site than would the reorganized path taken on a d4 metal. In both cases, a sulfur radical species should be formed in VI as long as a d3 metal is at the catalytic site. These radical sulfur species should be observable and a radical aggregation of sulfurs could be formed. An example of a d3 metal would be the cobalt-molybdenum catalyst prior to sulfiding or, possibly, an exposed Mo3+ (if the ion exists with that charge). One can also consider the involvement of a d2 metal in the manner of the d4 and d3 metals. For the d2 metal, a strong metal-sulfur bond results in the product fragment which should be the most difficult to break in the series of cases. Such a metallic species should become rapidly poisoned due to the difficulty in obtaining rapid sulfur turnover.
The Desulfurization of Thiophene and the Met hylt hiophenes Although the focus of the previous discussion had been on the role of the catalytically active metal ion in the surface, the model can be employed to compare the hydrodesulfurization of thiophene and the monomethylthiophenes, all of which have been studied experimentally by Desikan and Amberg.g Simple Huckel theory can be used since the three molecules have very similar P electron configurations describable in a straightforward manner, and since the number of B bonds being formed or broken will be the same in all three cases. It is further assumed that the strengths of these u bonds are alike in the three thiophenes. Therefore, the changes in P electron energies and charge distributions can be presumed to govern the differences in the chemistries of the thiophenes in the hydrodesulfurization reaction. Two ways of representing the effect of the methyl group on the P electron energies and charges in the thiophenes were examined.l0 In the inductive representation of the
352
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978
TABLE I: Huckel Parameters Useda
4x1 = a t h,P, S
c,
Grin
(methyl carbon, ductive) Cring (methyl carbon, heteroatom) CH, (heteroatom) Metal d,, Metal d,, a
TABLE 11: Experimentala and Theoretical Results for the Hydrodesulfurization of Thiophene, ZMethylthiophene, and 3-Methylthiophene
Oxy = kXyP
Atom
h,
1.0 0.1 -0.5
kx v
2.0
-1.35
2-Methyl- 3-MethylThiophene thiophene thiophene
s-c,, 0.8
-0.2 - 1.25
Anthony J. Duben
C-CH,, 0.7 d,,S, 0.25 d,,S, i 0.1
References 8 and 10.
methyl, the ring carbon to which it is bound has a Coulomb integral made more positive by -0.50 since the methyl is an electron donor. Any delocalization between the methyl group and the ring is ignored. In the heteroatom representation, the methyl group is considered to act as a single atom which adds two T electrons to the six already in the ring. The methyl heteroatom has an inductive effect of -0.20 on its neighbor, an exchange parameter of 0.70 with the ring carbon, and a Coulomb integral of a 2.00. The other Huckel parameters (including those for the metal) are the same as before. In those cases in which two effects are acting on the same carbon, the net change is taken to be the sum of the two separate modifications. For example, the a carbon to which the methyl is attached in 2-methylthiophene experiences an inductive effect of 0.10 from the sulfur and -0.50 from the methyl in the inductive representation. The net change will be -0.40 for that carbon. All of the Huckel parameters are collected in Table I. Huckel molecular orbital calculations were performed on thiophene and on the methylthiophenes in both the inductive and heteroatom models. The orbital energies of the methylthiophenes are raised with respect to thiophene as is expected since the methyl is an electron-donating group. In the heteroatom representation, the methyl group has an insignificant weighting in the unoccupied antibonding orbitals which will form H bonds with the metal d,, and d,. orbitals. In comparing these three molecules in the process of hydrodesulfurization, the rate-limiting step is presumed to be the protonation leading to the first carbon-sulfur bond breaking. Rupture of this bond should occur after protonation of the ring, which, in turn, requires the adsorption of the sulfur heterocycle on the metal ion. The breaking of the second carbon-sulfur bond is probably not as difficult if the known ease of the desulfurization of mercaptans as compared to thiophenes has its expected applicability here. The appropriate T bonded configurations in the mechanism are I-IV in Figure 4. The methyl group is at carbon 3 in I for 3-methylthiophene. The methyl group is at carbon 4 in I for 2-methylthiophene. When Huckel calculations are performed on configurations I-IV using the methylthiophenes, great similarities in the results for 2-methylthiophene and for 3-methylthiophene are found regardless of whether the inductive representation or the heteroatom representation is used for the methyl group. Positions where charge densities and bond orders are high in one representation are the same in the other. The same sort of thing is true with respect to energy changes from one configuration to another. Hence, it seems that the inclusion of the methyl into the H electron system has less influence than the inductive effects of the substituent. In the inductive representation, then, the adsorptions of 3-methylthiophene and 2-methylthiophene (I to 11) have
+
Molar concentration in H, carrier gas, % Rate, p mol converted/ g of catalyst/s at 225 " C Rate at 170 C Rate constants, 225 C Rate constants, 170 "C Activation energy for desulfurization, exptl, kcal Activation energy, theor, I1 to I11 (using p = 70.56 kcal) O
1.67 18.8 0.8 11.3 0.5 24.4
10.7 1.1 9.8 1.0 i
31.9
Heat of adsorption, exptl, kcal 8.7 Energy of adsorption, theor, I to I1 (using p = 7 0.56 kcal) 46.8
1.09
27.8 2.7 27.8 2.7
1 . 6 18.3 i 1.8 18.8 * 1.7 25.4b 23.2c
k
1.00
28.5b 27.3'
0.5 11.0 f 1 . 2 11.0 i 1 . 2 31.1b 25.9'
a Reference 9. Heteroatom representation. ductive representation.
43.5& 42.5' In-
energies of 0.600 and 0.370, respectively. These energies are less than the 0.660 for thiophene itself. For both molecules there is a substantial reduction of the S-C1 bond order to nearly zero. However, the two a carbons do not share equally in the acquisition of negative charge. The charge on the carbon C1 is -0.85 and -0.80 for 3methylthiophene and 2-methylthiophene, respectively. This is larger than the -0.74 charge on the same carbon when thiophene is adsorbed. On the other a carbon, C4, which is closer to the point of methyl substitution, the charge is -0.30 in 3-methylthiophene and -0.34 in 2methylthiophene. Based on the charge density, it can be expected that C4 will be a much less likely spot for the initial proton attack on the ring. The proton attack on the ring a t C1 proceeds with localization energies of 0.390 for 3-methylthiophene and 0.33P for 2-methylthiophene, both less than the 0.450 found for thiophene itself. This is consistent with the higher charges found at this carbon in the adsorbed species (11). These localization energies would then be the activation energies for the reaction. Interpretation of the experimental data taken from ref 9 and given in Table I1 requires a certain amount of precision is specifying the quantities to be compared. Presumably initial rates are reported at the two temperatures considered. These rates would be directly proportional to the rate constants (which are more appropriate quantities for comparison) if the rate law is the same for all of the molecules and if the molar concentrations of the three thiophenes in the H2 carrier gas are the same. However, in ref 9, the molar concentrations of the thiophenes are not the same. Hence, it becomes necessary to use the rates and concentrations to obtain effect rate constants for each molecule at each temperature. Since hydrogen appears to be in large excess, it would seem that pseudo-first-order kinetics would be an appropriate description of the kinetics of desulfurization. Then, if the rates are initial rates, the rate constants will be given by the quotient of the rates and concentrations. In this particular case, the modifications are not severe since the relative magnitudes of the rate constants are in the same
Hydrodesulfurization of Thiophene and the Monomethylthiophenes
order as for the rates themselves. The experimental kinetic data will be interpreted using the Arrhenius rate equation
k
= A exp(-Eact/RT)
The Journal of Physical Chemistry, Vol. 82, No. 3, 1978 353
f
(1)
where Eactis the activation energy, A is the frequency factor, and h is the rate constant. In making comparisons among similar reactions, it is frequently assumed that the factor A is the same for the molecules being considered. If this were so, the ratio of relative rate constants for two molecules would be a simple function of the activation energies:
2-methyl thiophene 3-rnethylthi O p k W
thiophene
Comparison of the relative rate constants with the experimental activation energies shows that eq 2 is inappropriate. The experimental activation energies would lead one to expect rate constants in the order 2-methylthiophene > 3-methylthiophene > thiophene. The same expectation is also obtained from examination of the theoretical activation energies (for I1 to 111). The rate constants or initial rates themselves put 3-methylthiophene a t the head of the list with thiophene and 2-methylthiophene at the bottom. Since the orders of the experimental and theoretical activation energies agree, it is apparent that the frequency factor must be considered further in interpreting the data. An indication of the relative magnitudes of the frequency factors can be best determined by using the theoretical activation energies. This is because the frequency factors determined from the experimental activation energies have inherent errors so large that they are almost meaningless due to the propagation of errors from the experimental activation energies. If the Huckel parameter p is taken to be 70.56 kcal obtained by correlating the Huckel transition energy of thiophene with its lowest UV band ,3 one finds from substituting into eq 1 that the frequency factors are related as thiophene > 3-methylthiophene > 2-methylthiophene with relative orders of magnitude of roughly 1000:100:1 in either the inductive or the heteroatom representations for the methyl group. Since there is a considerable difference in the charges on the 01 carbons of the adsorbed methylthiophenes resulting in the likelihood of only one of them being subject to attack (as contrasted with thiophene in which both a carbons have equal chances of being attacked), one can expect that the methylthiophenes should have lower frequency factors. However, this would not explain the order obtained involving orders of magnitude differences. A hint as to what may govern the relative order in frequency factors may be found in noting that the energies liberated in going from I to I1 (adsorption of the free thiophenic molecule on a metal ion in a proper orientation to react) are in the relative order of the frequency factors with the energy for the adsorption of 3-methylthiophene being closer to that of thiophene than to that of 2methylthiophene. These energies of adsorption are not the same as those reported by Desikan and Ambergg who believed that the heats they measured were for a flat adsorption of the thiophene on the catalyst. Their interpretation of their experimental data is reasonable since they found the same heat of adsorption for the two methylthiophenes. If the methylthiophenes undergo a flat adsorption, the methyl group's electron-donating ability should not be distinguishable with respect to position on the ring; only the gross effect of the methyl group should be seen as it affects the ring as a whole. The experimental adsorption energy of thiophene itself differs from that for
Figure 6. Activated chemisorption of thiophenes on the surface.
the methylthiophenes presumably because it lacks this electron-donating ability associated with methyl substitution. The energies in going from I to I1 are for the formation of an oriented thiophene in a configuration which is suitable for activation by the metal and thereby liable to attack. This is different from the adsorption considered by Desikan and Amberg. The energy of adsorption of the thiophenic molecules in an oriented manner is modified by the electron-donating influence of the methyl group. The methyl substituent will oppose the transfer of electrons from the metal to the thiophene ring. The stronger effect of the methyl in the a position rather than in the p position may be explained by noting that the a carbon can form a weak bond to the metal d,, and is thus a receptor of electrons. The inductive effect of the methyl in an a position would make accepting electrons by this carbon more difficult. In this respect, it does not seem necessary to presume the involvement of a purely geometric steric effect in analyzing the difference in the rates of hydrodesulfurization of the two methylthiophenes. Recent experimentsll indicate that steric effects are not particularly important. Since Desikan and Amberg did not observe large differences in heats of adsorption, it would seem that the oriented adsorptions (11) are a small fraction of the total number of molecules adsorbed. If the adsorption I to I1 is the result of an activated chemisorption of the thiophenes in which the heat of adsorption that Desikan and Amberg measure are only for the weakly bonded first step, then energy barriers for the activated chemisorption will be inversely related to the depth of the well (Figure 6) if the other quantities needed to describe the curves are the same. (The valley for the flat adsorption of the methylthiophenes is slightly lower than that for thiophene; it is not expected to be an important consideration.) In this manner, the energy of adsorption becbmes an indication of the portion of thiophenic molecules in the active perpendicular orientation; the greater this energy, the more molecules in the desired position. Then, thiophene should have the largest proportion of molecules favorably situated and have the largest frequency factor; and conversely, 2-methylthiophene should have the lowest frequency factor. Conclusions This investigation indicates that Huckel molecular orbital theory, as oversimplified as it is, applied to the Lipsch and Schuit model for hydrodesulfurization appears
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to be useful in interpreting the experimental kinetics of the reactions of thiophene and the monomethylthiophenes. From this work, it would seem that Mo3+is the active metal species based, in part, on the interpretation of ESCA experiments on sulfided Co-Mo catalysts. Cobalt is considered to be important is assisting the heterolytic breaking of the H2 bond so as to furnish hydride ions in the course of the reaction in order to prevent sulfide poisoning of the catalytic site. Theoretical activation energies are in good agreement with the experimental values. The Arrhenius frequency factors are interpretable using molecular orbital calculations to show that only a small fraction of the adsorbed thiophenes are in a catalytically active orientation, and that this proportion is determined by the energy of activated chemisorption onto the surface. In principle, this sort of technique could be extended to study other interesting catalytic systems. One that springs to mind immediately is that of the hydrodenitrification of pyridines which occur in significant amounts in synthetic crudes derived from shale or coal. The technology of hydrodenitrification is not as highly developed as that of hydrodesulfurization. It might be expected that investigations of this sort could lead to experimental investigations involving fewer false starts and dead-ends. Even in the more mature technologies such as hydrodesulfurization, investigations of this nature are of value. A more clear understanding of the limitations of the process on a molecular level could lead to a more realistic appreciation of the factors which are under control of the technologist in his attempts to improve the catalyst or the process. Hence, increasing the proportion of metal ions in a desirable electronic configuration should increase the activity of the catalyst. However, it would be difficult to overcome the problems associated with the numbers of suitably oriented molecules which are obtained from an activated chemisorption. The number of suitably oriented molecules (and the value of the frequency factor) appears to be, in large measure, a function of the electronic configuration of the substrate. If this is the case, it would seem that radical departures in the technology of hydrodesulfurization would be needed in order to overcome the problems associated with unfavorable frequency factors. The fact that frequency factors can vary over a number of orders of magnitude for such simple molecules as these
Anthony J. Duben
indicates that simulation studies in hydrodesulfurization should make allowance for the possibility of varying frequency factors among the different classes of sulfurcontaining molecules being included. To base the internal computation of rate constants on activation energies alone may not be the best procedure, especially when selectivities are being sought. Even in the experiments studied in this paper, there would be a crossover in selectivity for the hydrodesulfurization of these simple molecules. Since reaction temperatures change along the length of a reactor and, therefore, the rate constants will change as well, it would be worthwhile to have the best information possible in parameterizing a model. Theoretical calculations coupled with well chosen experiments could be useful in developing a data base for model building which would correspond better to actual physical experiment. Improved information would make a model not only a better interpretative tool, but also a more reliable predictive tool.
Acknowledgment. The author thanks Dr. Ernest L. Pollitzer of Universal Oil Products, Inc., where this work was done, for his encouragement and his support during the investigation. The author also thanks Professor John P. Lowe of the Pennsylvania State University for his comments and suggestions on a preliminary draft of this paper. References and Notes (1) Review articles and references therein: (a) P. C. H. Mitchell, "The Chemistry of Some Hydrodesulfurization Catalysts Containing Molybdenum", Climax Molybdenum Co., 1967; (b) S. C. Schuman and H. Shalit, Catal. Rev., 4, 245 (1970); (c) G. C. A. Schult and 6. C. Gates, AIChE J . , 19, 417 (1973); (d) C. H. Amberg, "Molybdenum in Hydrodesulfurization Catalysts" in "The First In,; ternational Conference on the Chemistry and Uses of Molybdenum, P. C. H. Mitchell, Ed., Climax Molybdenum Co., 1973, pp 180-187 (2) J. M. J. G. Lipsch and G. C. A. Schuit, J. Catal., 15, 179 (1969). (3) R. Zahradnik, Adv. Heferocycl. Chem., 5, 1 (1965). (4) U. Gelius, Theor. Chim. Acta, 27, 171 (1972). (5) R. Kebabcioglu and A. Mueller, Chem. Phys. Lett., 8, 59 (1971). (6) S. 0. Grim and L. J. Matienzo, Inorg. Chem., 14, 1014 (1975). (7) A. W. Armour, P. C. H. Mitchell, 6.Folkesson, and R. Larsson, "X-Ray Photoelectron (ESCA) Spectra of Some Molybdenum-containing Catalysts" in "The First International Conference on the Chemistry and Uses of Molvbdenum". P. C. H. Mitchell. Ed.. Climax Molybdenum Co., 1973, pp i92-193. (8) C. D. Garner, I. H. Hillier, J. Kendrick, and F. E. Mabbs, Nature (London), 258, 138 (1975). (9) P. Desikan and C. H. Ambera, Can. J. Chem., 41, 1966 (1963). (IO) A. Stretwieser, "Molecular OrbGI Theory for Organic Chemists", Wiley, New York, N. Y., 1961, p 135. (11) M. Zdrazil, Collect. Czech. Chem. Commun., 40, 3491 (1975).
