Acid catalyzed C-C bond forming to synthesize intermediates and energy carriers
Catalytic synthesis of carbon-carbon bonds from refractory alkanes as well as from light alcohols such as methanol are important pathways to generate olefins as well as hydrocarbons used for transportation fuels. The activity and selectivity of catalysts depends critically on the concentration, strength, and environment of the Bronsted acid sites active for these reactions. Lewis acid sites may play a direct and indirect role in this chemistry, indirect by facilitating hydride transfer reactions, direct in the oligomerization of light olefins through the insertion reactions into the metal-carbon bonds. We explore practical and mechanistic aspects of zeolite catalysis for an alkylation of isobutane with n-butene, oligomerization of alkenes, and the conversion of methanol to hydrocarbons. Linking the intrinsic properties of tailored zeolite catalysts with detailed mechanistic investigations is used to synthesize more selective and stable catalysts.
The alkylation of isobutane with n-butene is an elegant way of activating the non-reactive alkane with the help of the alkene and to transform it to a high-octane component important for clean gasoline. The currently used catalysts (concentrated sulfuric acid and HF) are corrosive liquids that were attempted to be replaced with benign solid catalysts for a long time. Using detailed kinetic measurements at very low olefin concentrations the mechanism of alkylation of isobutane with n-butene has been shown to be a subtle balance between the addition butene and the removal of the surface alkoxy groups with hydride transfer from isobutane. The best catalysts maximize the hydride transfer rate, while also having a maximum concentration of Brønsted acid sites. Zeolites with pores larger 0.6 nm have been shown to provide the steric requirements for hydride transfer. Introduction of La3+ cations close to Brønsted acid sites has been shown to lead to unprecedented activity and selectivity. The (inevitable) deactivation of the catalysts was shown to occur by the formation of strongly unsaturated hydrocarbon residues that accumulate during the alkylation process and block acid sites and by multiple alkylations at the outer surface of the particles preventing access to the micropores of the active catalyst. This combination of deactivation routes leads eventually to a sudden halt of the catalytic activity. We have shown that overall coke accumulates exponentially and is preferably removed in the early stages of formation. Understanding the nature of the coke allowed to develop a low temperature liquid phase and high temperature gas phase regeneration method that allowed stable operation of the catalysts for over 10 months producing more than 600 kg alkylate per kg catalyst, which is at least over 60 times superior to sulfuric acid. In cooperation with Süd-Chemie and Lurgi a new process for solid acid catalyzed alkylation has been developed. The catalysts have been optimized over the last years utilizing the insight about the unique nature of La3+ in promoting hydride transfer.
The MTO reaction on HZSM-5 has been clearly identified as autocatalysis mechanism, with mobile olefins and aromatic products in the zeolite pore acting as competing co-catalysts. Accordingly, two distinct reaction pathways, aromatics-based and olefin-based, are active for the production of ethene and propene from methanol over HZSM-5 under reaction conditions relevant to practical operations. The aromatics-based cycle starts with toluene as the lowest sufficiently active species, while a complete olefin methylation/cracking cycle begins its turnover with propene as the lowest sufficiently active species. The aromatics-based cycle produces ethene and propene with equal carbon based selectivities, while the olefin-based cycle favors C3+ olefins over ethene.
The co-existence of olefins and aromatics species in the zeolite pores leads to a competition between the two cycles for chemisorbed methanol. Therefore, total activity depend on the local activities of specific hydrocarbon species and methanol conversion. Co-processing of intermediates in each catalytic cycle of the same concentrations results in identical involvements in the turnover and in turn an identical impact on the product distribution, due to the comparable rate coefficients in each step of a cycle. While co-feeding lower substituted benzenes propagates the aromatics-based cycle, the olefins produced by the aromatics based cycle will subsequently propagate also the olefin based cycle. In turn, olefin homologation/cracking reactions are more important than the aromatics based cycle at higher methanol conversions, contributing to C3+ higher olefins formation irrespective of the aromatics or olefinic nature of co-feeds. On the other hand, the aromatics based and the olefin based cycle operate for ethene formation, and the dominant pathway for ethene formation depends to a large extent on the reaction conditions. With an aromatics-enriched feed and/or at low reaction temperatures, the aromatics based cycle contributed predominantly, while the olefin based cycle contributes significantly as well when a high reaction temperature is adopted (such as in the MT(O)P process). The results shown here also demonstrate the presence of a specific hydrogen transfer pathway involving chemisorbed methanol intermediates, which is significantly faster than classic hydride transfer between two olefinic species and has significant consequences for catalyst lifetime under MTO process conditions.
The role of parallel and sequential reactions in Brønsted acid catalyzed conversion of methanol to olefins on H-ZSM-5 was explored by comparing the catalysis in plug-flow (PFR) and fully back-mixed reactors (CSTR). Catalysts deactivated under homogeneous gas phase in the back-mixed reactor show unequivocally that in the early stages of the reaction the zeolite deactivates via blocking of individual Brønsted acid sites and not by coke induced impeding access to pores. While the two reactors led only to slight differences in product distribution, catalyst deactivation rates were drastically lower in the CSTR. H-ZSM-5 deactivated in the CSTR first rapidly and then at a much slower rate. During the initial phase, the rate was directly proportional to the methanol partial pressure and was caused by oxygen-containing surface species. These species were transformed to aromatic compounds with time on stream and the deactivation proceeded then via methylation of aromatic compounds, forming the typical coke species for MTO processes. The outer surface of the polycrystalline particles is virtually carbon free under these conditions. Formation of condensed aromatic species throughout the deactivation in voids between crystalline domains occurs as parallel reaction without affecting the deactivation kinetics.
A new hydrogen transfer pathway from methanol to carbenium ions during the conversion of methanol to olefins on H-ZSM5 has been identified as major pathway to aromatic molecules and alkanes in the early stage of the reaction. Low concentrations of formaldehyde in the reactants leads to hydride transfer products, i.e., higher concentrations of C6+ aliphatics and aromatics and lower concentrations of light olefins. Experiments on pure Lewis acidic (LAS)-MFI unequivocally showed that methanol and propene react on LAS to formaldehyde and propane. The novel hydride transfer pathway is linked to the reaction of olefins and methanol to paraffins and formaldehyde at LAS and the reaction of olefins with formaldehyde to aromatic molecules and paraffins on Brønsted acid sites (BAS). The importance of the pathways depend on the relative concentrations of BAS and LAS. Catalysts with a low LAS concentration produce less aromatics and paraffins. Conversely, varying BAS concentrations at a high BAS/LAS ratios hardly affects hydrogen transfer rates. At low BAS/LAS ratios, hydrogen transfer becomes important because the competitive adsorption of formaldehyde on BAS suppresses olefin methylation and cracking reactions.
Related publications
Common mechanistic aspects of liquid and solid acid catalyzed alkylation of isobutane with n-butene A. Feller and J. A. Lercher, J. Catal., 216, 313 (2003).
On the mechanism of solid acid catalyzed isobutane/butene alkylation over zeolite based catalysts A. Feller, A. Guzman, I. Zuazo and J. A. Lercher, J. Catal., 224, 80 (2004).
Stages of aging and deactivation of zeolite LaX in isobutane/2-butene alkylation C. Sievers, I. Zuazo, A. Guzman, R. Olindo, H. Syska and J. A. Lercher, J. Catal., 246, 315 (2007).
On the impact of co-feeding aromatics and olefins for the methanol to olefins reaction on HZSM-5 X. Sun, S. Müller, H. Shi, G.L. Haller, M. Sanchez-Sanchez, A. C. van Veen, J.A. Lercher, J. Catal., 314, 21 (2014).
On reaction pathways in the conversion of methanol to hydrocarbons on HZSM-5 X. Sun, S. Müller, H. Shi, G.L. Haller, M. Sanchez-Sanchez, A. C. van Veen, J.A. Lercher, J. Catal, 317, 185 (2014).
Coke formation and deactivation pathways on H-ZSM-5 in the conversion of methanol to olefins S. Müller, Y. Liu, M.Vishnuvarthan, X. Sun, A. C. van Veen, G. L. Haller, M. Sanchez-Sanchez, J. A. Lercher, J. Catal., 325, 48 (2015).