“…As expected, the latter (−65 kJ/mol) is slightly lower than that calculated earlier 9 for a single methanol molecule (−73 kJ/mol for the 3TOH cluster). This result agrees with one of the experimental heats of adsorption for methanol in H-ZSM5 zeolite reported in the literature, −63 kJ/mol, but it is lower than other two, −108 and −115 kJ/mol …”
Density functional theory is used to study the zeolite acid
catalyzed methanol dehydration to dimethyl ether.
Three different reaction pathways are proposed. In the first,
methanol adsorption and surface methoxy species
formation are the initial elementary steps for this reaction.
Subsequent dimethyl ether formation by reaction
of a new methanol molecule with the surface methoxy species takes
place. The second path involves the
simultaneous adsorption and activation of two methanol molecules with
formation of dimethyl ether and
water in one step. The third path involves also the simultaneous
adsorption and activation of two methanol
molecules. The difference is that, like in the first path,
initially a methoxy surface species will be formed
from dehydration of one of the methanol molecules, and this will be
followed by dimethyl ether formation.
The second path appears to be the preferred route for dimethyl
ether formation, since its activation barrier is
lower than the other two paths. The effect of making the zeolitic
cluster slightly more acidic (by lengthening
the Si−H bond distances) over the activation barriers of dimethyl
ether formation has been studied. Changes
on the order of 5 kJ/mol are observed. An analysis of the reaction
rate constants for the three reaction paths
of methanol dehydration is also presented.
“…As expected, the latter (−65 kJ/mol) is slightly lower than that calculated earlier 9 for a single methanol molecule (−73 kJ/mol for the 3TOH cluster). This result agrees with one of the experimental heats of adsorption for methanol in H-ZSM5 zeolite reported in the literature, −63 kJ/mol, but it is lower than other two, −108 and −115 kJ/mol …”
Density functional theory is used to study the zeolite acid
catalyzed methanol dehydration to dimethyl ether.
Three different reaction pathways are proposed. In the first,
methanol adsorption and surface methoxy species
formation are the initial elementary steps for this reaction.
Subsequent dimethyl ether formation by reaction
of a new methanol molecule with the surface methoxy species takes
place. The second path involves the
simultaneous adsorption and activation of two methanol molecules with
formation of dimethyl ether and
water in one step. The third path involves also the simultaneous
adsorption and activation of two methanol
molecules. The difference is that, like in the first path,
initially a methoxy surface species will be formed
from dehydration of one of the methanol molecules, and this will be
followed by dimethyl ether formation.
The second path appears to be the preferred route for dimethyl
ether formation, since its activation barrier is
lower than the other two paths. The effect of making the zeolitic
cluster slightly more acidic (by lengthening
the Si−H bond distances) over the activation barriers of dimethyl
ether formation has been studied. Changes
on the order of 5 kJ/mol are observed. An analysis of the reaction
rate constants for the three reaction paths
of methanol dehydration is also presented.
“…Further using the rule of Hess it is possible to evaluate the heat of adsorption, Δ Α H , from the heat of immersion, Δ l H , and heat of condensation, Δ L H , by the relation (−)(Δ A H ) = (−)Δ 1 H + (−)Δ L H (such data has been collected in Table ). Our data on n -alkane/ZSM-5 compares reasonably well with the values reported by Stach for ethane and butane and Messow et al for C 5 to C 17 n -alkane (see Table footnote). This again establishes the basic soundness of our adopted model and calculations.…”
Section: Resultssupporting
confidence: 91%
“…Data on heat of immersion studies are available, from which the idea of heat of adsorption could also be gained. To obtain heats of immersion, Δ l H , Messow et al took into account the limiting adsorption values n s 3 Trends of heats of adsorption of n -alkane of different zeolite surfaces as a function of their carbon number. …”
The role of electronic polarizabilities in dealing with the phenomenon of molecular adsorption and catalysis
by zeolites has been discussed in this work. Making use of the principles of periodic minimal surfaces of
crystalline materials and rules of atomic connectivities of zeolite structures, an expression has been derived
to evaluate heats of adsorption of hydrocarbons interacting with the surfaces of zeolites that reads ΔH =
αM·β·γ + θf, where αM is the average polarizability of hydrocarbons, β and θf are the constants, and γ the
average curvature of the zeolite surface. After determining the polarizabilities of hydrocarbons by a quantum
mechanical method and establishing the magnitudes of the curvatures of surfaces of zeolites by differential
geometrical approach, the formalism has been applied to evaluate heats of adsorption of C1−C8 hydrocarbons
on the typical surfaces of MOR, FAU, ZSM-5, and silcalite zeolites. The agreement between theoretical
and experimental values of heats of adsorptions of paraffins, naphthenes, olefins, and aromatics has been
found to be very good and compares fairly well with those obtained from experimental methods as well as
derived by other theoretical methods. The applicability of the data so generated has been demonstrated in
estimating the driving force of cracking (activation energy, E) following the Evans−Polyani procedure in
the case of n-paraffins interacting with ZSM-5 surface. The data so generated has been used to explain
negative activation energy for cracking observed for n-hexadecane and higher alkanes (C16 effect).
“… 17 , 37 , 38 , 40 The H-bond lengths of CH 3 OH and CH 3 SH concur with previous theoretical DFT/BLYP results, 22 reporting 1.52 and 2.25 Å for the H surf ···O(CH 3 OH) and H surf ···S(CH 3 SH) bond lengths, respectively. The methanol adsorption enthalpies are similar to the most recent theoretical studies also using the ONIOM quantum mechanics/molecular mechanics (QM/MM) approach (−24.4 37 and −27.4 kcal/mol 16 ) and fall within the range of experimental methanol adsorption energies (−15 to −27 kcal/mol 41 , 42 ).…”
Methyl mercaptan—a harmful
impurity in natural gas—may
be selectively converted into H2S and hydrocarbons [methyl
mercaptan to hydrocarbon (M2TH) process], using zeolite catalysts.
When M2TH is compared with the well-known MTH (methanol to hydrocarbons)
process, significant differences emerge, essentially regarding the
formation and distribution of products. Density functional theory
(DFT) and Born–Oppenheimer molecular dynamics (BOMD) were employed
to reveal possible origins for the experimentally observed differences.
We established a close similarity between DFT intrinsic (electronic)
reaction profiles in the stepwise mechanism of methanol and mercaptan
dehydration, although no variance in reactivity was revealed. BOMD
simulations at the experimental temperature of 823 K reveal rapid
hydrogen abstraction from the methyl group in mercaptan, adsorbed
in the zeolite cavity in the presence of the methoxy intermediate.
The formation of •CH2SH radical is 10 times faster
than that of •CH2OH at the same temperature. The
varied reactivity of methanol and mercaptan in MTH and M2TH processes,
respectively, can therefore first be attributed to very rapid hydrogen
abstraction in mercaptan, which occurs in the zeolite cavity, following
the formation of surface methoxy.
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