Wenmin Zhang

Characterization of the acidity of ultrastable Y, mordenite, and ZSM-12 via NH3-stepwise temperature programmed desorption and Fourier transform infrared spectroscopy

The acidity of ultrastable Y (USY), mordenite, and ZSM-12 of variable Si/Al ratios has been characterized by coupling NH3-stepwise temperature programmed desorption (STPD) and FT-IR. The former technique allows us to quantify accurately the ammonia chemisorbed on acid sites of different strengths. In contrast to other acidic oxide catalysts, it was found that protonated zeolites possess distinct limits of acid strength and that chemisorbed ammonia desorbs from each type of site only within particular temperature ranges. It was observed that for zeolites with Si/Al ratios smaller than about 20 the number of ammonia molecules chemisorbed on both Lewis and Bronsted sites is smaller than the total number of Al atoms present in the zeolite. However, an increase in the Si/Al ratio above this threshold results in a 1:1 relation between the total number of acid sites and the amount of ammonia adsorbed. It was found that for mordenite and ZSM-12 an increase of the dealumination severity results in a decrease of the acid site strength. For USY the acid strength decreased with steam treatment, but then increased with acid leaching. The FT-IR studies revealed that for the protonated USY, Lewis-bound ammonia is evolved within each temperature stage. For mordenite, the Lewis sites are associated with desorption up to 250°C. The non-dealuminated ZSM-12 (Si/Al=35) sample possesses a relatively small number of Lewis sites, whereas the ZSM-12 samples with higher Si/Al ratios had only Bronsted acid sites. A peak deconvolution procedure was performed in order to decompose accurately the complex hydroxyl region of USY and mordenite and was used to separate the Lewis peak from structural vibrations for ZSM-12. As a result, important information concerning the nature of the aluminum species present in these zeolites (extraframework species, framework species present within the side pockets of mordenite, framework species present within the β cages of USY, etc.) was obtained.

Acidity of dealuminated β-zeolites via coupled nh3-stepwise temperature programmed desorption (STPD) and FT-IR spectroscopy

Abstract Ammonia stepwise temperature programmed desorption (STPD) and FT-IR spectroscopy have been used to study the acidity of dealuminated β-zeolites with Si/Al ratios in the range of 14.5 to 132. A carefully optimized temperature profile starting at 150 °C (in order to exclude physisorbed NH 3 ) revealed five peaks at about 180 °C, 250 °C, 350 °C, 440 °C, and 540 °C. Our investigations indicate that the first two peaks correspond to Lewis-type acidity of low strength and that the last three peaks correspond to Bronsted-type acidity of increasing strength. A one-to-one relation between the total number of ammonia molecules desorbed in the above temperature range and that of the Al atoms exists. High strength Lewis sites were not observed probably because the extent of dehydroxylation in the above temperature range was minimal. By increasing the level of dealumination, it was found that the relative population of strong Bronsted sites increases in comparison with the weak ones. The present stepwise temperature programmed desorption experiments coupled with FT-IR provide a very accurate tool for the quantitative measurement of the strength of the zeolite acid sites.

Dealuminated zeolite-based composite catalysts for reforming of an industrial naphthene-rich feedstock

Abstract Dealuminated ZSM-12, zeolite β and their composites with γ-Al 2 O 3 as a matrix were evaluated for the reforming of an industrial naphtha (rich in naphthenes). The reactions were carried out at temperatures in the 270–470°C range. It was found that dealuminated ZSM-12 demonstrates unique time-on-stream stability for the reactions investigated. This behavior is a combined result of: (a) its pore structure which does not favor coke formation, and (b) the balance of its acidity. Zeolites with channel intersections, which are slightly larger than the zeolite aperture do not favor coke formation. Our results demonstrated that the composite catalysts produce more gasoline-range hydrocarbons and show much better time-on-stream behavior than conventional γ-Al 2 O 3 catalysts. Fresh and deactivated catalysts were characterized by XRD, NH 3 -stepwise TPD, TGA, and FTIR. Soluble carbonaceous deposits analyzed by high-resolution GC/MS are mostly paraffinic in nature. The paraffin deposits present over the Pt/γ-Al 2 O 3 catalyst were of higher molecular weight than those over the zeolite catalysts. We propose that, at relatively lower reaction temperatures, the catalyst deactivates via a successive alkylation type of mechanism in which carbonaceous coking precursors propagate through the continuous addition of olefinic intermediates to carbenium/carbonium ions.

On the exceptional time-on-stream stability of HZSM-12 zeolite: relation between zeolite pore structure and activity

ZSM-12 and several other 12-membered ring large-pore zeolites have been tested for the reforming of naphthenic hydrocarbon mixtures. It was found that ZSM-12 possesses a surprisingly higher coking resistance than other large pore zeolites tested such as USY, L-zeolite, mordenite, and β=zeolite for reforming of hydrocarbon mixtures. This superior performance is due to the unique non-interconnecting tubular-like linear channels of ZSM-12, which do not allow trapping/accumulation of coking precursors. ZSM-12 zeolite also demonstrated excellent structural stability even under severe acid dealumination. From this work, we found that the decrease of the aluminum content of a zeolite is not sufficient to ensure low rates of coke deposition. We also concluded that zeolites with channel intersections (cavities) of comparable size with the zeolite apertures do not favor coke formation. For these types of zeolites the strong acid sites carry out other acid-catalyzed reactions, rather than forming coke. In contrast, zeolites with relatively large supercages are inherently favorable to coking reactions, which in turn lead to the fast deactivation. The appropriate combination of the zeolite pore structure and acidity (controlled via dealumination) showed superior TOS behavior (time-stable activity and product selectivities). For zeolites which are susceptible to coking due to pore structure, the increase of the Brønsted acid strength results in fast deactivation. Contrary to what one would commonly expect and previous reports, we found that one-dimensional zeolites, such as, ZSM-12, can exhibit significantly higher tolerance to coking than multidirectional zeolites.

Study of the oxidative methylation of acetonitrile to acrylonitrile with CH4 over Li/MgO catalysts

Abstract The oxidative methylation of acetonitrile to acrylonitrile with methane for temperatures in the range 550–730°C over Li/MgO follows a radical mechanism. The reaction proceeds via the formation of radicals at the α-carbon of acetonitrile and methyl radicals from methane. The coupling of these radicals leads to propionitrile which is further transformed to acrylonitrile via oxidative dehydrogenation. Experimental evidences indicate that the reaction is Langmuir–Hinselwood. The Li+O− surface sites of Li/MgO are the active centers for the activation of both methane and acetonitrile. Oxygen is absolutely necessary for the formation of the corresponding radicals from methane and acetonitrile but it must be provided at a controllable manner in order to avoid undesired oxidation reaction of nitriles. The decomposition of acetonitrile which would lead to CH4 and HCN does not take place. However, the increase of the nitrile chain length favors the breaking of the C–C bond between the cyanide group and the α-carbon of the corresponding nitrile.

Oxidative Methylation of Acetonitrile to Acrylonitrile with CH4

Several basic catalysts have been tested for the oxidative methylation of acetonitrile to acrylonitrile. We found that Li/MgO catalysts with nominal lithium content in the vicinity of 25 wt % are effective for this reaction. Such optimum performance is associated with the maximum concentration of the [Li+O-] surface centers of Li/MgO. The coupling reaction takes place via radical mechanism between methane and the α-carbon of acetonitrile and, leads to propionitrile, which is further transformed via oxidative dehydrogenation to acrylonitrile. Experimental evidences indicate that the reaction is Langmuir-Hinselwood. It was concluded that the above active centers are primarily responsible for the advantageous behavior observed and not the basicity of the catalysts. Other supports and catalysts which were found to be very active by other researchers in generating methyl radicals from methane, proved to be ineffective for the transformation of acetonitrile to acrylonitrile. For example mono- and bimetallic combinations of alkali metals on Sm2O3, La2O3, CaO, and Bi2O3 catalyzed primarily the oxidation of acetonitrile to COx.