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Hydroconversion of n-hexadecane on Pt/silica-alumina catalysts: Effect of metal loading and support acidity on bifunctional and hydrogenolytic activity
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
Istituto per lo Studio dei Materiali Nanostrutturati, ISMN CNR, Palermo, Italy.
Istituto per lo Studio dei Materiali Nanostrutturati, ISMN CNR, Palermo, Italy.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
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2014 (English)In: Applied Catalysis A: General, ISSN 0926-860X, E-ISSN 1873-3875, Vol. 469, 328-339 p.Article in journal (Refereed) Published
Abstract [en]

Bifunctional catalysts based on platinum and amorphous silica-alumina were studied in the hydroconversion of n-hexadecane. The influence of platinum loading and support acidity on activity and selectivity were assessed. The contribution of hydrogenolysis reactions on top of bifunctional hydrocracking was shown to depend not only on metal loading, but also on the effect of support acidity on the intrinsic activity of the platinum sites. The yield of cracking products, and their linear alkane fraction, increased with metal loading, while the isomerization yield was practically independent of the metal content. On a support of high Bronsted acidity, the rate of formation of methane was proportional to the platinum surface area, indicating that dernethylation occurred by metal-cataly ed hydrogenolysis. On the other hand, the methane site-time yield was one order of magnitude lower on a catalyst with negligible Bronsted acidity. Pt-catalyzed hydrogenolysis was also investigated during selective poisoning of acid sites by cofeeding pyridine and comparing the effect of hydrogen partial pressure on reaction rates. In the presence of pyridine, total hydroconversion activity was reduced by one order of magnitude while rates to methane and linear cracking products remained relatively high. These observations indicate that acid sites do not intervene in the mechanism, but that support acidity affects the hydrogenolytic activity of platinum sites.

Place, publisher, year, edition, pages
2014. Vol. 469, 328-339 p.
Keyword [en]
Bifunctional hydrocracking, Hexadecane, Hydroconversion, Hydrogenolysis, Platinum, Silica-alumina
National Category
Other Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-129966DOI: 10.1016/j.apcata.2013.09.048ISI: 000329266500040Scopus ID: 2-s2.0-84886736656OAI: oai:DiVA.org:kth-129966DiVA: diva2:653928
Funder
Swedish Energy Agency
Note

QC 20140131. Updated from submitted to published.

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Hydroconversion of model Fischer‑Tropsch wax over noble metal/silica-alumina catalysts
Open this publication in new window or tab >>Hydroconversion of model Fischer‑Tropsch wax over noble metal/silica-alumina catalysts
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Synthetic fuels produced using the Fischer-Tropsch technology will play an important role in the future of the transportation sector. The Fischer-Tropsch synthesis (FTS) allows converting synthesis gas (CO + H2) into fuels of outstanding quality. The synthesis gas can be obtained from different carbon sources: natural gas, coal and biomass. In order to maximize the yield of middle distillates, the process is carried out in two main steps: the FT-synthesis that produces long-chain hydrocarbons (waxes) and a hydrocracking step, to selectively convert the waxes into fuels. Diesel produced by this process is characterized by excellent combustion properties and reduced harmful tailpipe emissions compared to conventional diesel.

Due to the growing interest in synthetic fuel production, from the industry and the academia, and to the peculiar characteristics of the Fischer-Tropsch products, research in hydrocracking has received renewed attention. Catalysts for the hydrocracking of long-chain paraffins have been the subject of the present work, which is the summary of four scientific publications.

Noble metals supported on acid carriers have been compared, especially for what regards the mechanisms through which hydrocracking proceeds. The catalysts were synthesized and characterized by various techniques, including N2 physisorption, H2 chemisorption, TEM, pyridine adsorption FTIR, ammonia TPD, etc. It was shown that catalytic activity is mainly dependent on the acid support used; that selectivity is strongly dependent on conversion, high conversion favoring highly branched cracking products. Two main reaction routes were observed: bifunctional hydrocracking and hydrogenolytic cracking. Platinum-containing catalysts showed high selectivity towards the latter, while palladium/silica-alumina behaved as pure bifunctional catalysts. Catalyst deactivation was investigated and initial sintering of metal particles was observed. Coking was also a cause of deactivation. Formation of coke deposits was highly dependent on the metal loading of the catalysts. Metal loading also influenced catalyst selectivity, especially in the case of platinum/silica-alumina catalysts. Monofunctional hydrogenolysis on the platinum particles, superimposed to the bifunctional mechanism was observed. This route increased selectivity towards linear hydrocarbons and methane, with increasing amounts of platinum. The specific rate of hydrogenolysis was constant for different loadings of platinum on the same acid silica-alumina support. Using a different, less acid, support negatively affected the hydrogenolytic activity of the platinum catalytic sites. It was concluded that metal-support interactions might play an important role in the catalytic properties of platinum surfaces.

This work has contributed to increasing the knowledge about hydrocracking of long-chain alkanes and pointed out some features that might have practical interest in the application of this technology to synthetic-fuel production.

Abstract [sv]

Syntetiska drivmedel tillverkade genom Fischer-Tropsch teknologin kommer i framtiden att ha en betydande roll för transportsektorn. Fischer-Tropsch syntesen (FTS) möjliggör omvandling av syntesgas (CO + H2) till högkvalitativa bränslen. Syntesgasen kan erhållas från olika kolkällor: naturgas, kol och biomassa. För att maximera utbytet av medeldestillat, utförs processen i två huvudsteg: FT-syntes som producerar långa kolväten (vaxer) och ett hydrokrackning steg, för att selektivt omvandla vaxerna till bränslen. Diesel som produceras med denna process kännetecknas av utmärkta förbränningsegenskaper och ger upphov till minskade utsläpp av skadliga ämnen jämfört med vanlig diesel.

På grund av det växande intresset för syntetiska bränslen, både från industrin och den akademiska världen, och av de speciella egenskaperna hos Fischer-Tropsch-produkter, har forskningen i vätekrackning fått förnyad uppmärksamhet. Ämnet för detta arbete, som är en sammanfattning av fyra vetenskapliga publikationer, är katalysatorer för hydrokrackning av långkedjiga paraffiner.

Ädelmetaller uppburna på sura bärare har jämförts, särskilt vad gäller vätekrackningsmekanismer. Katalysatorerna preparerades och karaktäriserades med hjälp av olika tekniker, bland andra N2 fysisorption, H2 kemisorption, TEM, pyridin adsorption FTIR, ammoniak TPD, etc. Det visade sig att den katalytiska aktiviteten är främst beroende av surheten hos bärarmaterialet, att selektivitet är starkt beroende av omsättningen, där hög omsättning gynnar flergrenade krackningsprodukter. Två huvudsakliga reaktionsvägar observerades: bifunktionell vätekrackning och hydrogenolytisk crackning. Platinakatalysatorer visade hög selektivitet mot det senare, medan katalysatorer med palladium på kiseloxid-aluminiumoxid uppträdde som rena bifunktionella katalysatorer. Katalysatordeaktivering undersöktes och sintring av metallpartiklar observerades. Koksning var också en orsak till deaktivering. Koksbildning var starkt beroende av metallhalten i katalysatorerna. Metallhalten påverkade också selektivitet, särskilt för platina-kiseloxid-aluminiumoxidkatalysatorer. Monofunktionellt hydrogenolys på platinapartiklarna, observerades utöver den bifunktionella mekanismen. Med denna reaktionsväg ökade selektivitet mot linjära kolväten och metan, med ökande platinahalter på katalysator. Den specifika reaktionshastigheten för hydrogenolys var konstant för olika platinahalter på en sur kiseloxid-aluminiumoxidbärare. Den hydrogenolytiska aktiviteten hos platina katalytiska säten påverkas negativt när en mindre sur bärare användes. Slutsatsen var att interaktioner mellan metallen och bäraren kan spela en viktig roll för de katalytiska egenskaperna hos platina ytor.

Detta arbete har bidragit till att öka kunskapen om vätekrackning av långkedjiga alkaner och påpekade vissa funktioner som kan ha praktiskt intresse vid tillämpningen av denna teknik för produktionen av syntetiska bränslen.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. xi, 97 p.
Series
Trita-CHE-Report, ISSN 1654-1081 ; 2013:38
Keyword
hydroconversion, Fischer-Tropsch wax, n-hexadecane, bifunctional hydrocracking, hydrogenolysis, silica-alumina, platinum, palladium
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-129968 (URN)978-91-7501-855-3 (ISBN)
Public defence
2013-10-11, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20131007

Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2013-10-07Bibliographically approved

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