Richard G. Herman

Catalytic synthesis of methanol from COH2: I. Phase composition, electronic properties, and activities of the Cu/ZnO/M2O3 catalysts

The low pressure methanol synthesis catalysts CuZnO, Cu/ZnO/Al2O3, and Cu/ZnO/Cr2O3 were found to contain a new compound identified as a CuI solution in ZnO, which is also an active component of the above catalysts. Combined X-ray diffraction, optical, and XPS—Auger studies are presented that describe the formation, electronic structure, and surface composition of these catalysts. In particular, the surface of the working catalyst is free of carbon, both in the presence and absence of CO2 in the feed gas. A synthesis mechanism is proposed whereby the CuI centers nondissociatively chemisorb and activate carbon monoxide and the ZnO surface activates hydrogen. Catalyst deactivation in COH2 mixture is explained as the reduction of CuI to inactive copper metal, while the rate enhancing effect of O2, H2O, and CO2 is due to the maintenance of an oxidation potential high enough to keep the copper in the active CuI state. No special pore distribution or presence of crystalline phases such as spinels is necessary for selectivity of the CuZnO catalyst to the formation of methanol.

Catalytic synthesis of methanol from COH2: IV. The effects of carbon dioxide

The effects of carbon dioxide on the catalytic synthesis of methanol over the copper-zinc oxide catalysts were investigated for CO2/CO/H2 ratios between 0/30/70 and 30/0/70. A maximum synthesis rate was observed-at CO2/CO/H2 = 2/28/70. At lower concentrations of CO2 the catalyst is deactivated by overreduction and at higher concentrations of CO2 the synthesis is retarded by a strong adsorption of this gas. A kinetic model is presented which quantitatively describes the observed patterns in the indicated range of synthesis gas compositions and at temperatures between 225 and 250 °C. This model is consistent with all physical characteristics of the CuZnO catalysts and corroborates earlier findings that an intermediate oxidation state of the catalyst is its active state. The adsorption enthalpies and entropies for the reactants indicate that carbon dioxide is strongly bound and immobile while carbon monoxide and hydrogen are chemisorbed with intermediate strength and experience a considerable mobility in the adsorbed layer. At concentrations of CO2 greater than 10%, methane is a side product. Mechanistic implications of this finding are that there is a nonselective pathway parallel to the CO hydrogenation; this pathway may involve formate and methoxy intermediates.

Higher alcohol and oxygenate synthesis over cesium-doped Cu/ZnO catalysts

The synthesis of higher (C+2) alcohols and esters has been studied over cesium-doped CuZnO catalysts. Under higher alcohol synthesis conditions, e.g., 583 K, 7.6 MPa, and gas hourly space velocity = 3260 liters (STP)/kg cat/hr with a H2CO = 0.45 synthesis gas, the presence of cesium promoted the formation of higher oxygenates, especially 2-methyl-1-propanol. The yields of products passed through distinct maxima at cesium nominal concentrations of 0.3–0.5%. These nominal concentrations generated optimum surface cesium concentrations of 15–25%. Under the reaction conditions employed, the principal role of cesium was to increase the ethanol synthesis rate and to provide an even greater enhancement in the rate of ethanol conversion to 1-propanol and subsequently to higher alcohols. To obtain insight into the mechanism of the carbon chain growth, e.g., C1 → C2, C2 → C3, and linear versus branched carbon chain growth, a 13C-NMR study of the C2C4 products formed over CuZnO and 0.4 mol% Cs/Cu/ZnO catalysts was performed. Separate injections into the COH2 synthesis gas of methanol and ethanol with natural-abundance 13C and enriched by 13C in specific positions showed that (i) lower alcohols were incorporated into the synthesis to form higher alcohols; (ii) carbon chain growth occurred in a stepwise manner dominated by the addition of oxygenated C1 intermediates at the β carbons of the oxygenated Cn (n ≥ 2) intermediates but also proceeding via linear addition Cn + C1 (n ≥ 1); and (iii) the presence of cesium had a dramatic effect on the reaction mechanism and promoted greatly the synthesis rates. The mechanistic effects of the alkali dopant were most pronounced in the C2 → C3 step. Over the CuZnO catalyst, injection of ethanol produced 1-propanol via linear chain growth, i.e., CH313CH2OH + COH2 → CH313CH2CH2OH. The presence of Cs effected a mechanistic switch and promoted β-carbon addition, CH313CH2OH + COH2 → 13CH3CH2CH2OH. The position of the 13C label in the CH3 group of propanol provides evidence for retention of oxygen associated with the C1 intermediate, formed from COH2, and loss of oxygen associated with the 13CH2OH group of ethanol. Mechanistically, such a retention is favored by a β-ketoalkoxide intermediate that is bonded to the cesium centers via its anionic oxygen. This unique mechanism is termed herein as aldol coupling with oxygen retention reversal and is specific to the presence of the cesium salt dopant. Higher alcohol synthetic steps C2 + C2 and Cn + Cm (n ≥ 3, m = 1, 2, 3) were also analyzed. Both oxygen retention reversal and normal oxygen retention were observed in coupling reactions leading to the higher-molecular-weight products (m + n > 3), and this observation is attributed to steric effects favoring the cis conformation of the β-alkoxide followed by rejection of either of its two oxygens.

Methanol and C2 oxygenate synthesis over cesium doped Cu/ZnO and Cu/ZnO/Al2O3 catalysts: A study of selectivity and 13C incorporation patterns

Cesium formate significantly promotes methanol and C2 oxygenate (methyl formate MF and ethanol EtOH) synthesis over the CuZnO catalyst. 13C NMR analysis of the product obtained from 13CH3OH and 12COH2 shows that MF is formed by direct carbonylation of methanol and EtOH is formed by coupling of oxygenated C1 intermediates originating from methanol. The C1 coupling mechanism overrides various CO insertion paths to ethanol over the present Cs/Cu/ZnO catalyst. The kinetic significance of C1 aldehydic species such as adsorbed formyl and formaldehyde is supported by experimental evidence and theoretical calculations.

Discrimination among Carbonate Minerals by Raman Spectroscopy Using the Laser Microprobe

Raman spectroscopy has been used to distinguish the individual carbonate minerals belonging to the calcite, dolomite, and aragonite structural groups. With the use of the in situ laser microprobe technique, it is demonstrated that particle-size effects do not hinder the mineral identification and that high precision in line position is achieved. Spectra can be obtained from samples in any form and provide rapid, nondestructive analyses on a microscopic scale of the hard-to-discriminate carbonate minerals.

Higher alcohol and oxygenate synthesis over Cs/Cu/ZnO/M2O3 (M=Al, Cr) catalysts

Surface doping of Cu/ZnO/M{sub 2}O{sub 3} (M = Al, Cr) catalysts prepared from hydrotalcite precursors with cesium (Cs/Cu/ZnO/M{sub 2}O{sub 3}) significantly enhanced to the alcohol synthesis rate under higher alcohol synthesis conditions. With respect to the unsupported Cs/Cu/ZnO catalyst, the product selectivity of the Cs/Cu/ZnO/Al{sub 2}O{sub 3} catalyst was shifted toward methanol, while the Cs/Cu/ZnO/Cr{sub 2}O{sub 3} catalyst maintained a high selectivity toward C{sub 2}{sup +} alcohols. The presence of cesium in the Cu/ZnO/M{sub 2}O{sub 3} catalysts inhibited the synthesis of dimethyl ether. Comparison of the product distributions obtained over the Cs/Cu/ZnO/M{sub 2}O{sub 3} catalysts with those observed over the Cs/Cu/ZnO catalysts indicates that the function of the Cs/Cu/ZnO/Cr{sub 2}O{sub 3} catalyst is similar to that of the Cs/Cu/ZnO in that higher alcohols are synthesized by a stepwise carbon chain growth via a unique aldol coupling with oxygen retention reversal mechanism. The Al{sub 2}O{sub 3}-based catalysts undergo complex structural changes that probably cause occlusion of the Cs dopant, thus resulting in low selectivity to higher alcohols while retaining high activity toward methanol. 23 refs.

Development of active oxide catalysts for the direct oxidation of methane to formaldehyde

Formaldehyde is currently produced from methane by a three-step process involving H2/CO synthesis gas and methanol as intermediates, and development of a single-step process would have great economic incentive for producing this commodity chemical. A historical perspective is presented here in regard to the research carried out with heterogeneous metal oxide catalysts in attempts to achieve selective oxidative conversion of methane to formaldehyde. The concepts employed, both chemical and engineering, are described, and these include dual redox promoters and double-bed catalysts. More recent work in this laboratory has found V2O5/SiO2 catalysts to be very active partial oxidation catalysts. The space-time yield of and selectivity toward formaldehyde are improved by the presence of steam in the methane/air reactant mixture, and an attractive feature of the product mixture is the low quantity of carbon dioxide produced. Space-time yields of >1.2 kg CH2O/kg catalyst per h have been achieved.

Precursors of the copper-zinc oxide methanol synthesis catalysts

Abstract The coprecipitated hydroxycarbonate precursor of the methanol synthesis and shift reaction catalyst based on 30 at.% copper and 70 at.% zinc oxide, which was previously reported to be a mixture of hydrozincite Zn5(CO3)2(OH)6 and rosasite (Cu,Zn)2(CO3)(OH)2 (R. G. Herman, K. Klier, G. W. Simmons, B. P. Finn, J. B. Bulko, and T. P. Kobylinski, J. Catal. 56, 407, 1979) or a single-phase hydrozincite (G. Petrini, F. Montino, A. Bossi, and G. Gaybassi, in “Studies in Surface Science and Catalysis. Preparation of Catalysis III” (G. Poncelet, P. Grange, and P. A. Jacobs, Eds.), Vol. 16, p. 735. Elsevier, The Netherlands, 1983), is herein shown to be a single-phase aurichalcite (Cu0.3Zn0.7)5(CO3)2(OH)6. The orthorhombic B2212 aurichalcite is crystallograpically distinct from the monoclinic C 2 m hydrozincite, although these two compounds have the same ratio of metal ions to carbonate and hydroxyl anions. Both aurichalcite and hydrozincite are chemically and structurally distinct from the monoclinic P2 1 a rosasite. The earlier erroneous assignments are attributed to the structural similarity of the three hydroxycarbonates in question. An energy-dispersive characteristic X-ray emission analysis of individual particles in the scanning transmission electron microscope reveals a uniform distribution of copper and zinc at the analytical concentration Cu Zn = 30 70 . Precursors with less than 30% copper consist of mixtures of aurichalcite and hydrozincite.

Kinetic modelling of higher alcohol synthesis over alkali-promoted Cu/ZnO and MoS2 catalysts

Abstract Kinetic networks have been developed for the synthesis products that are observed to be formed over alkali-promoted Cu/ZnO- and MoS 2 -based higher alcohol synthesis catalysts. Alcohols are synthesized over these two types of catalysts by two different mechanisms having different kinetically important steps. Linear alcohols are formed by CO insertion over the alkali/MoS 2 catalysts, while the alkali/Cu/ZnO catalysts promote the formation of branched alcohols via a β-addition process that has a kinetically limiting C 1 → C 2 step. The derived kinetic parameters can be used to predict the influence of reaction conditions, e.g. temperature, pressure, H 2 /CO ratio, and GHSV, on product yields and selectivities in differential and integral reactors.

Selective oxidation of methane with air over silica catalysts

Partial oxidation of methane by oxygen to form formaldehyde, carbon oxides, and C2 products (ethane and ethene) has been studied over silica catalyst supports (fumed Cabosil and Grace 636 silica gel) in the 630–780 °C temperature range under ambient pressure. The silica catalysts exhibit high space time yields (at low conversions) for methane partial oxidation to formaldehyde, and the C2 hydrocarbons were found to be parallel products with formaldehyde. Short residence times enhanced both the C2 hydrocarbons and formaldehyde selectivities over the carbon oxides even within the differential reactor regime at 780 °C. This suggests that the formaldehyde did not originate from methyl radicals, but rather from methoxy complexes formed upon the direct chemisorption of methane at the silica surface at high temperature. Very high formaldehyde space time yields (e.g., 812 g/kg cat h at the gas hourly space velocity = 560 000 ℓ(NTP)/kg cat h) could be obtained over the silica gel catalyst at 780 °C with a methane/air mixture of 1.5/1. These yields greatly surpass those reported for silicas earlier, as well as those over many other catalysts. Low CO2 yields were observed under these reaction conditions, and the selectivities to formaldehyde and C2 hydrocarbons were 28.0 and 38.8%, respectively, at a methane conversion of 0.7%. A reaction mechanism was proposed for the methane activation over the silica surface based on the present studies, which can explain the product distribution patterns (specifically the parallel formation of formaldehyde and C2 hydrocarbons).