Four-Carbon Segmented Discrete Hydrocracking of Long-Chain Paraffins in MTT Channels Following a Pore-Mouth Mechanism

Abstract

Hydrocracking of long-chain paraffins is a key process in petroleum refining. A fundamental understanding of the reaction on the bifunctional catalyst is of great importance for controllable cracking. In this work, a mechanism study on the hydrocracking of long-chain paraffins catalyzed by Pt/ZSM-23 has been carried out. The distribution and accessibility of acid sites on MTT channels are purposefully adjusted. Based on reasonable simplification, the reaction data are analyzed to reveal the shape selectivity, reaction path, and catalytic mechanism. The applicability of the pore-mouth mechanism in this system has been carefully verified by experiments. Both protonated cyclopropane (PCP) and β-scission paths occurred simultaneously at the pore mouths, accounting for 60 and 40%, respectively. The proportions of PCP and β-scission paths with different chain lengths inserted in the pore are quantitatively calculated, which are multimodal. Shape selectivity of the teardrop-shaped MTT skeletal structure is investigated by the joint experimental−theoretical method. The insertion of the paraffin chain in the MTT channel is discretely segmented by integral multiples of four sequential carbon atoms starting at one end of the chain, which is followed by cracking through the mixed PCP and β-scission paths at pore mouths.

Toward Superior Hydroisomerization Catalysts through Thermodynamic Optimization

“…Neither set of the histograms exhibits the pronounced M-like signature for α-methyl alkane hydrocracking with maxima at C 4 and C N-4 and with a pronounced minimum at C N/2 , the center. 25,54 Although the hydrocracking product histograms reported for MTT-and TON-type zeolites are markedly different from those expected for α-methyl alkane hydrocracking, in that they miss the consistent minimum at the center of the histogram and the signature maxima at both ends of the histogram (as frequently reported for MFI-type zeolite 25,52,54 ), MTT-and TON-type zeolites yield remarkably few i-alkane and remarkably many n-alkane hydrocracking products, 56,59,62 which could still point to contributions by α-methyl alkane hydrocracking. One reason for a historically small i-alkane fraction in the hydrocracking products is a historically low peak isomer yield (Table 3), suggesting transport or hydrogenation rate limitations (Figure 1).…”

“…Hydrocracking equilibrium mixtures of α,α- (◯), α,γ- (□) dimethyl, and α-monomethyl (×) alkanes exhibit a signature variation of the height of the center of the histogram with feed n -alkane carbon number after normalization to N = 12 by multiplication with ( N -5)/(12-5). The experimental variation with carbon number of the normalized center height of histograms obtained by hydrocracking n -alkanes on MTT- (●) ,, and on TON- (■) ,,, type zeolites (Table ) suggests exclusively hydrocracking of α,α- and of α,γ-dimethyl alkanes, respectively.…”

“…Since α-monomethyl alkane and α,α- or α,γ-dimethyl alkane hydrocracking both maximize the yield of C 4 and C N ‑4 products (Figure , Supporting Information), a combination of α-methyl alkene and of α,α- or α,γ-dimethyl alkene cracking would leave its signature in the product histograms through maximizing the C 4 and C N ‑4 yield, and through high C 3 and C N ‑3 yields as compared to exclusive α,α- or α,γ-dimethyl alkene cracking. Histograms combining these characteristic features have been reported ,, and were attributed to combined α-methyl, α,α-dimethyl, and α,γ-dimethyl alkene cracking, but alternate explanations for these features have remained in play. ,, Interestingly, these M-shaped histograms, arguably characteristic of combined α-methyl, α,α-dimethyl, and α,γ-dimethyl alkene cracking, were reported for MFI-type zeolites, i.e., for adsorbed phases with nooks and crannies that favor formation of alkenes with two proximate branches (Table ) , and not for adsorbed phases with consistently narrow (∼0.50 nm wide) pores that impede the formation of α,α- and α,γ-dimethyl alkenes. − , …”

“…9,25,55 Interestingly, these M-shaped histograms, arguably characteristic of combined α-methyl, α,αdimethyl, and α,γ-dimethyl alkene cracking, were reported for MFI-type zeolites, i.e., for adsorbed phases with nooks and crannies that favor formation of alkenes with two proximate branches (Table 2) 9,55 and not for adsorbed phases with consistently narrow (∼0.50 nm wide) pores that impede the formation of α,αand α,γ-dimethyl alkenes. [33][34][35]56 There is also a plurality of opinions on where alkenes isomerize and crack. Some consider it established that isomerization and cracking occur throughout 0.50−0.58 nm wide pores.…”

“…There is also a plurality of opinions on where alkenes isomerize and crack. Some consider it established that isomerization and cracking occur throughout 0.50–0.58 nm wide pores. ,, Others assume that these pores are inert except for a small sliver near the pore ends, as a basis for “pore mouth” and “key-lock” imagery. ,,,,,− Some have proposed compromises. , Others ignore any assumed inertness and implicitly assume pores partake fully in isomerization and cracking. ,,,,,,,, …”