Shiqi Bao

Activity and selectivity of precipitated iron Fischer-Tropsch catalysts

Abstract Low-temperature (230°C), slurry phase Fischer-Tropsch synthesis (FTS) was conducted with precipitated iron-silicon catalysts under industrially relevant conditions (flow=3.1 Nl h −1 g-Fe −1 , H 2 :CO=0.7, P=1.31 MPa). The effects of activation gas (hydrogen, carbon monoxide, or syngas) and promoters (potassium and copper) on activity and selectivity were explored. Optimum potassium promotion was 4–5 at%, relative to iron. Promotion with copper lowered the reduction temperature and increased FTS activity, regardless of the activation gas used. Carbon monoxide activation gave the highest activity for a 100Fe/ 4.4Si/5.2K catalyst (atomic percent, relative to iron) while syngas activation was superior for a 100Fe/4.4Si/2.6Cu/5.2K catalyst. Selectivity of the FTS product was not affected by the activation gas employed or copper promotion; however, potassium promotion increased wax and alkene selectivity. A syngas activated 100Fe/4.4Si/2.6Cu/5.2K catalyst gave the best overall performance at 230°C. Alkene selectivity was >75% for the C 2 -C 11 fraction, methane selectivity was 12 + selectivity was >70 wt%.

Fischer–Tropsch synthesis: induction and steady-state activity of high-alpha potassium promoted iron catalysts

Abstract Iron Fischer–Tropsch synthesis (FTS) catalysts with different potassium loadings showed different induction periods during which the conversion increased from a low initial level to a peak value before declining to attain a lower stable activity at the same reaction conditions. A lower K loading produced a slightly higher peak conversion and a shorter induction period. Although, the induction period and the peak conversion were slightly dependent on the K loading for the iron catalyst, the stabilized conversions and the stabilization periods were independent of potassium content. The C 2 C 4 olefin to paraffin ratio of the gaseous products and the CO 2 selectivity did not change significantly as the potassium content increased from 5 to 10%. An increase in reaction temperature produced a new induction period and a higher conversion than was obtained before the reaction temperature was increased. The H 2 /CO ratio also had an important influence on FTS conversions. Increasing the H 2 /CO ratio in the feed gas lowered the H 2 utilization. A higher H 2 /CO ratio feedstock gas produced lower FTS catalyst activity compared to a low H 2 /CO ratio gas.

Fischer-Tropsch synthesis. Evidence for two chain growth mechanisms

The results show that n-pentanol serves to initiate Fischer-Tropsch synthesis reactions. Product accumulation in the CSTR is not adequate to explain the deviation from a constant14C activity/mole with increasing carbon number for alkane products. A second Fischer-Tropsch synthesis mechanism that produces only alkanes is needed to explain the deviation of the C activity/mole with increasing carbon number for n-alkanes. Furthermore, the two chain growth pathways must be completely independent without the possibility of a carbon number species that is common to both mechanisms. It is suggested that the pathway that incorporates added14C labeled alcohol has an oxygen containing surface intermediate while the other reaction pathway involves an “oxygen-free” reaction intermediate.

Role of CO2 in the initiation of chain growth and alcohol formation during the Fischer-Tropsch synthesis

Abstract 14CO2 added to the syngas serves to initiate chain growth to produce both oxygenates and hydrocarbons. This observation prompted a closer examination of the production of alcohols during Fischer-Tropsch synthesis with precipitated iron catalysts containing either silica or alumina and various levels of the potassium promoter. Surprisingly, the catalysts containing alumina produced more alcohols than the silica containing catalysts. The oxygenates fraction increased with increasing potassium content. At high potassium loadings, propanal becomes the dominant C3 oxygenate, suggesting that aldehyde, and not alcohol, is a primary product.

IRON FISCHER-TROPSCH CATALYSIS — PROPERTIES OF AN ULTRAFINE IRON OXIDE CATALYST

A commercial Fe oxide with a particle size of 3 nm is now available. The FT requires considerable time on stream before steady state conditions are attained. Since it is desirable to obtain FT data for the smaller ultrafine Fe oxide catalysts at larger times on steam, data for operation up to 6 months were collected using slurry phase. Results show that the ultrafine Fe oxide maintain catalytic activity for a 150-day operating period. Addition of 0.5% K increased the activity; after 56 days, the activity had declined to and below that of unpromoted catalyst. Neither the unpromoted nor K-promoted catalyst exhibited good selectivity for alkenes.

Methanol to gasoline: 14C tracer studies of the conversion of methanol/higher alcohol mixtures over ZSM-5

Abstract A series of experiments using a ZSM-5 catalyst and a reactor feed of either methanol mixed with carbon-14 labeled ethanol, 1-propanol or 1-pentanol or carbon-14 labeled methanol mixed with ethanol, 1-propanol or 1-pentanol was carried out. For conversions at 180, 200 or 300°C, the carbon-14 distribution indicated nearly complete equilibration of the carbons of the labeled and unlabeled alcohols. In the present study, the data show that isotope equilibration occurs within carbon number fractions and among compounds in each of the C 2 -C 4 fractions. These data demonstrate also that carbon isotope equilibration within C 3 + compounds is rapid compared to chain growth; this conclusion appears to apply for earlier tracer studies as well. Methanol is also the dominant source of the methane formed during conversion of the alcohol mixtures. Methanol is also the dominant source of the C 2 products, except when ethanol is the second alcohol.

Investigation of hydrogen transfer from carbon-14 ring labeled methylcyclohexane during the methanol-to-gasoline reaction

The selective conversion of methanol to hydrocarbons using a ZSM-5 catalysts was a significant discovery. The products follow an Anderson-Schulz-Flory distribution but with at least one important exception, the products above C/sub 10/ make an insignificant contribution to total product yield. The mechanism for the polymerization reaction is complex and three of the more widely accepted reaction pathways involve different reaction intermediates: (i) carbocations, e.g., CH/sub 3//sup +/ (ii) carbene (:CH/sub 2/), and (iii) oxonium ion (e.g., (CH/sub 3/)/sub 3/O/sup +/). Common to the above three reaction pathways is the final step(s), dehydrogenation to produce aromatics. There is strong evidence to support the view that the formation of one aromatic molecule is accompanied by the formation of three alkane molecules. To obtain information about the contribution of this reaction to the methanol-to-gasoline synthesis, a mixture of methanol and /sup 14/C-labeled methylcyclohexane has been converted using an HZSM-5 catalyst. Determining the distribution of /sup 14/C in the products would permit at least a semiquantitative measure of the importance of cyclohexane dehydrogenation as well as cycloalkane isomerization and cracking in the overall reaction work. 7 references.

Role of CO2 oxygenates and alkenes in the initiation of chain growth during the Fischer-Tropsch synthesis

Publisher Summary This chapter discusses the role of CO2 oxygenates and alkenes in the initiation of chain growth during the Fischer–Tropsch synthesis (FTS), which involves the production of many hydrocarbons and oxygenate products from CO and H2; this is especially true for synthesis using an iron catalyst. Alcohols produce hydrocarbons during the FTS that are not consistent with a simple initiation mechanism. Experimental results show that CO2 is produced directly from the alcohol and not by the reverse carbonylation reaction. CO2 also initiates chain growth in the FTS. It is proposed that the intermediate responsible for chain initiation in FTS using an iron catalyst is the same intermediate as encountered in the water-gas-shift reaction. The FTS mechanism with an iron based catalyst involves an oxygenate species that can be derived from both CO and CO2. The data generated during the studies, involving the addition of CO2 or alcohol, is consistent with a reaction mechanism that involves reactions as shown above for the alcohol. Thus, the intermediate that is involved in the water-gas-shift reaction is postulated to be an initiator for chain growth for the FTS reaction on the iron catalyst.

Conversion of [14C]methanol and propane mixtures with H-ZSM-5

One of the major discoveries in zeolite catalysis was the conversion of methanol to gasoline using H-ZSM-5 catalysts. While there have been many mechanistic studies, the reaction pathway and the C{sub 1} species involved in the reaction remain uncertain. The proposed mechanisms may be classified into three groups based upon the required intermediate: (a) mechanisms utilizing carbene (:CH{sub 2}), (b) mechanisms utilizing cations (e.g., CH{sub 3}{sup +}), and (c) mechanisms utilizing trimethyloxonium ions ((CH{sub 3}){sub 3}O{sup +}) or similar oxygen-containing species. Preliminary {sup 14}C tracer studies provided evidence that was consistent with the oxonium ion intermediate. Refinements in the {sup 14}C analytical procedures showed that the {sup 14}C added in propanol had completely scrambled with the methanol carbon and the data could therefore not define the intermediate species. Considering the importance of experiments with the methanol-propane mixture to an understanding of the methanol to gasoline conversion, the author has carried out studies using {sup 14}CH{sub 3}OH and unlabeled propane.

Fischer-Tropsch synthesis: Comparison of product selectivity and 14c labeled ethanol incorporation at one and seven atmosphere conditions

Abstract 14 C tracer studies using labeled ethanol indicate that the relative radioactivity/mole in hydrocarbon products is constant to carbon number 20. This result is consistent with (1) the two carbon units of ethanol remaining intact while on the catalyst surface and (2) that an adsorbed ethanol derived species serves to initiate chain growth but that it does not participate in chain propogation. The selectivity data obtained in a one atmosphere plug flow reactor are in excellent agreement with those obtained at seven atmospheres in a stirred tank reactor.