G. Giannetto

Hydroisomerization and hydrocracking of n-heptane on Pth zeolites. Effect of the porosity and of the distribution of metallic and acid sites.

Abstract The catalytic properties of PtHY, PtHZSM5 and PtH mordenite catalysts are controlled by the number of their acid sites (n A , number of acid sites on which the adsorption heat of NH 3 is greater than 100 kJ mol −1 ) and their hydrogenating sites (n Pt ). The activity of PtHY catalysts changes with n Pt /n A as can be expected from the classical bifunctional mechanism : it increases with n Pt /n A for the low values of this ratio, then reaches a plateau for n Pt /n A ⩾ 0.03. For low values of n Pt /n A , monobranched, multibranched isomers and cracking products are formed simultaneously but, for n Pt /n A ⩾ 0.17, n-heptane transforms successively into the three families of products. With PtHZSM5 catalysts, the same trend can be observed concerning the changes of the activity as a function of n Pt /n A . However the changes in the reaction scheme as a function of n Pt /n A are different : cracking remains the main reaction even at high values of n Pt /n A ; multibranched isomers are found in very low concentration. This can be attributed to a slower migration of the olefinic intermediates. Moreover, particular selectivities are found which are explained by a good matching between some intermediates and the space available near the acid sites. With PtH mordenite, the activity changes differently with n Pt /n A . In particular, it decreases for n Pt /n A > 0.015. This can be attributed to a partial pore blockage by platinum or by coke, leaving only a small part of the catalyst active. This is confirmed by the fact that the selectivities are the same as those of a PtHY zeolite with a n Pt /n A ratio 10–20 times lower.

Transformation of LPG into Aromatic Hydrocarbons and Hydrogen over Zeolite Catalysts

Abstract A way to increase the value of LPG cut from petroleum feedstocks is its direct transformation to H2 and aromatic products; these aromatic products, BTX—essentially benzene (B), toluene (T), and C, -aromatics (X)—can be used as raw material for the petrochemical industry or as a blending mixture to enhance the octane number of gasoline. However, these transformations require high temperatures. Thermodynamic data show that the conversion of paraffins into aromatics is favored by increasing the length of the chain, and that aromatics are favored in relation to olefins (Table 1) [1,2]. Whereas aromatization of propane and higher paraffins can be carried out at temperatures lower than 500°C, transformation of ethane, and especially that of methane, requires much higher temperatures. This is experimentally supported by the transformation of various hydrocarbons, at constant temperature and space velocity. For instance, over H-[All-ZSM-5, butane and isobutane react four times faster than propane and 100...

Transformation of acetophenone over Pd HFAU catalysts—reaction scheme

The transformation of acetophenone was carried out over a 0.5 wt.% Pd HFAU catalyst (Si/Al=17) under the following conditions: flow reactor, 250°C, pressure of ketone and hydrogen equal to 0.8 and 0.2 bar, respectively. The reaction products were identified either by comparison in GC with reference compounds: benzene (B), ethylbenzene (EB), styrene (EB=), cumene (IPB), isopropylbenzene (IPB=) and benzoic acid (BA) or through GC/MS coupling: 1,3-diphenylbutane (DPB), 1,3-diphenylbutenes (DPB=), 1,3-diphenylbutan-1-one (DPBO), 1,3-diphenylbut-2-ene-1-one (DPBO=) and 2,4-diphenyl-3-methylpentenes (DPMP=). These products are formed through three main reaction paths. DPBO= results from successive aldolisation of acetophenone and dehydration of the resulting alcohol over the protonic sites of the zeolite, DPBO from the hydrogenation of 1,3-diphenylbuten-1-one over the Pd sites (path 1). The formation of EB= involves hydrogenation of acetophenone followed by dehydration of the produced alcohol; EB results from EB= hydrogenation (path 2). DPB= can result from EB= dimerization (path 2) or from hydrogenation of DPBO followed by dehydration of the resulting alcohol (path 2). These reactions are similar to those observed during acetone and cyclohexanone transformation over bifunctional catalysts. In these reactions no alcohol intermediate is observed, which shows that alcohol dehydration is much faster than aldolisation and hydrogenation steps. The third reaction path which leads mainly to IPB, IPB= and BA, plays a significant role in acetophenone transformation whereas this path was very slow in acetone and cyclohexanone transformations. BA and IPB= result from acid cracking of DPBO=, IPB from hydrogenation of IPB=. IPB= undergoes also dimerization into DPMP= and IPB undergoes dealkylation into B over the protonic sites of the HFAU zeolite.

Transformation of acetone over a 0.4PtHMFI(60) catalyst. Reaction scheme

The transformation of acetone was carried out over a 0.4 wt% PtHMFI catalyst (SiAl = 60) under the following conditions: flow reactor, 160°C, pressures of acetone and hydrogen equal to 0.75 and 0.25 bar, respectively. Methylisobutylketone, propane and traces of mesityloxide are observed as primary products while the other main products: 2-methylpentane and diisobutylketone result from secondary transformation of methylisobutylketone. The reactivity of the reaction products and of probable intermediates: diacetone alcohol, isopropanol and propene was compared to that of acetone, which allows us to establish the complete scheme of acetone transformation. Acetone is competitively transformed through bifunctional catalysis into methylisobutylketone and into propane. The limiting step of methylisobutylketone formation is acetone aldolisation over the acid sites of the catalyst while that of propane formation is acetone hydrogenation over platinum sites. Methylisobutylketone undergoes the same competitive bifunctional transformations leading to diisobutylketone (limiting step: acid coaldolisation of acetone and of methylisobutylketone) and to 2-methylpentane (limiting step: hydrogenation of methylisobutylketone).

Effect of the metallic/acid site (nPt/nA) ratio on the transformation of acetone towards methyl isobutyl ketone

Methyl isobutyl ketone (MIBK) was synthesized from acetone (Ac) and hydrogen over Pt-HZSM5 bifunctional catalysts. The reaction was carried out at 160°C, atmospheric pressure, and with a PH2/PAc molar ratio = 0.33, using a fixed bed and dynamic flow reactor. The results show that catalytic properties and coke formation largely depend on the ratio between the number of accessible hydro-dehydrogenation sites and the number of theoretical acidic sites (nPt/nA).

Phenol alkylation with methanol: effect of sodium content and ammonia selective poisoning of an HY zeolite

Sodium exchange and ammonia selective poisoning of the acid sites of an HY zeolite (Si/Al=20) were carried out and their effects on the catalytic properties for the alkylation of phenol with methanol (200‡C, 1 atm and N2/reactants molar ratio of 4) were evaluated. Results show that the reaction is highly sensitive to the number and strength of the acid sites of the catalyst. A decrease in the number of acid sites by sodium exchange of the protons or by ammonia selective poisoning produces important changes in the selectivity of the reaction. In fact, a high increase in the anisole/cresol ratio is observed as the percentage of exchanged sodium in the zeolite increases, while the ammonia selective poisoning shows that at low desorption temperatures (⩽250‡C) only anisole is formed while at higher desorption temperatures both anisole and cresols were observed. These results show that anisole formation requires sites with lower acid strength compared to those necessary for cresol formation.

Acetone transformation into methyl isobutyl ketone over Pt/HMFI catalysts. IV. Effect of density and strength of the acidic sites

Acetone transformation into methyl isobutyl ketone (MIBK) was studied using a fixed-bed dynamic reactor at 160°C, 1 atm pressure and acetone/H2 molar ratio=3. The reaction was carried out over Pt/HMFI bifunctional catalysts, with 0.30 wt% of platinum which was supported over three aluminosilicates (Si/Al ratio=40, 95 and 160) and a borosilicate (Si/B ratio=44) with similar dispersion. The results show that catalytic properties depend greatly on density and strength of the acidic sites of the catalysts. Moreover, the limiting step of the MIBK synthesis reaction (aldolization of two acetone molecules) is carried out over the acidic sites of the aluminosilicates, but not over those of the borosilicate, which considerably affects reaction selectivity.

Effect of aluminum and gallium concentration on the crystallization rate of TPA/MFI zeolites synthesized with MeNH2 in the absence of inorganic cations

Abstract The synthesis of [M]-ZSM-5 zeolites (M = Ga, Al, Ga + Al) in the absence of inorganic cations and with tetrapropylammonium (TPA) as a template agent and methylamine (MA) as a complexing and mobilizing agent, was carried out at 185°C, under autogenous pressure and rocking agitation, using a low reactive aerogel. The samples were characterized by XRD, i.r., SEM, and 27 Al and 71 Ga MAS n.m.r. The results show that the efficiency of the aluminum and gallium addition into the zeolite structure is almost 100%, but the morphology and the size of the crystals as well as the crystallization rate of the zeolites are modified by the chemical composition of the synthesis gel. The presence of aluminum strongly reduces the crystallization rate of metallosilicates with MFI-type structure. The higher the concentration of aluminum, the higher the decrease. This might be related to the lower stability of the aluminum monomeric species.

A new way to obtain acid or bifunctional catalysts. II. Straightforward calcination of as-synthesized [Ga,A1]-ZSM-5 zeolites obtained from alkali-free media

Abstract Straightforward calcination of as-synthesized alkali-free [Ga]-ZSM-5 zeolites leads to the formation of pure acidic or bifunctional gallium-promoted acidic MFI catalysts, depending upon the calcination temperature (Tc). At Tc≤530°C, the organic molecules are removed without any significant structural modification and zeolite behaves as a pure acidic catalyst. At Tc ≥ 700°C, this removal is accompanied by an important degalliation of the zeolite structure producing bifunctional catalysts (acid and Ga promoted) showing properties for propane and naphthene aromatization similar to those of the catalysts prepared by conventional impregnation method which involve relatively long and costly intermediate steps.

Preparation of acidic or bifunctional catalysts by means of straightforward calcination of as-synthesized [Ga]-ZSM-5 zeolites obtained from alkali-free media. Propane aromatization

Propane aromatization (530°C, 1 atm) was used as a reaction model to evaluate the effect of the calcination temperature on the catalytic properties of an as-synthesized [Ga1.3]-ZSM-5 zeolite obtained from alkali-free media and calcined at two different temperatures: 530°C (C-530) and 750°C (C-750). Results show that in spite of its lower acidity, C-750 is more active and selective toward aromatics than C-530. This is probably due to the fact that at higher temperature the decomposition of organic compounds used during the zeolite synthesis is accompanied by a partial degalliation of the zeolitic support leading to the production of a bifunctional x“Ga2O3” /H-[Gay-ZSM-5(2x+y=1.3)catalyst.