F. Alvarez

Structure-activity relationship in zeolites

Abstract In this review we will try to give an integrated view of the relation between the structure and the catalytic behaviour of zeolitic materials, covering both the compositional and the structural aspects. Due to the amount of work that has been done in this wide area of zeolite catalysis this will, necessarily, be an incomplete although, hopefully, unbiased work. The paper will refer, not only to the relation that can be observed with natural and as-synthesised zeolites, but also to the wide range of techniques that are currently available for the ‘tuning’ of the catalytic properties of zeolitic materials. A special emphasis will be placed in all the aspects concerning shape selective catalysis, which is certainly the most striking form of a structure-activity relationship in heterogeneous catalysis.

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).

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.

Transformation of cyclohexanone on PtHZSM5 catalysts — reaction scheme

Abstract The transformation of cyclohexanone was carried out on PtHZSM5 catalysts under the following conditions: flow reactor, 473 K, pressures of cyclohexanone and hydrogen equal to 0.25 and 0.75 bar respectively. Six families of products were identified by GC or GC-MS analysis: C6 cyclic hydrocarbons 1, C12 bicyclic hydrocarbons 2 (e.g., cyclohexylcyclohexene), cyclohexenylcyclohexanone 3, cyclohexylcyclohexanone 4, phenylcyclohexanone 5, tricyclic ketones 6 (e.g., biscyclohexenylcyclohexanone). A reaction scheme is proposed to explain the formation of these products. Compounds 1 would result from the following steps: hydrogenation of cyclohexanone (probably in the enol form) on Pt sites, dehydration of cyclohexanol on the acid sites, hydrogenation or dehydrogenation of cyclohexene on Pt sites. Compounds 2 are mainly formed through successive transformations of 4: hydrogenation, dehydration…; 3 results from aldolisation of cyclohexanone followed by dehydration of the resulting alcohol, 4 from hydrogenation of 3, 5 from dehydrogenation of 3. The compounds 6 result from aldolisation of 3 with cyclohexanone followed by dehydration, hydrogenation and dehydrogenation steps. The dehydration of alcohols is much more rapid than aldolisation and hydrogenation—dehydrogenation steps. On a 0.2 PtHZSM5 catalyst with a platinum dispersion greater than 70%, aldolisation is slower than hydrogenation—dehydrogenation steps. The deactivation of the catalyst affects more the acid sites than the metallic ones.

Synthesis of methyl isobutyl ketone using Pt–H[Al]ZSM5 bifunctional catalysts. III. Effect of the nPt/nA parameter on the xPt–H[Al]ZSM5(y) catalysts

Synthesis of 4-methyl-2-pentanone, better known as methyl isobutyl ketone (MIBK), from propanone (Ac) was studied in a fixed-bed and a flow reactor at 160 °C, 1 atm and an H2/Ac molar ratio equal to 0.33, using Pt-H[Al]ZSM5 bifunctional catalysts with variable platinum percentage and Si/Al ratio. The results show that initial total activity to all measured products at t = 0, per acidic site (Ao/nA), residual activity (Ar = A265/A0, ratio of activity after 265 min of stream and the initial total activity to all measured products at t = 0) and initial formation rate of each product per acidic site (Ro/nA) largely depend on the relationship between the number of hydro-dehydrogenating metallic sites and the number of theoretical acidic sites (nPt/nA) present in the catalysts used.

Hydroisomerization and Hydrocracking of Alkanes. 5. Hydroisomerization and Hydrocracking of N-Hexane and N-Heptane on PtHY Catalysts. Effect of the Distribution of Metallic and ACID Sites.

The activity, the stability and the selectivity of a series of bifunctional PtHY catalysts containing 0.02 to 1.5 wt% platinum and having Si/Al atomic ratios of 3 or 9 were compared for n-hexane and n-heptane transformations at 250°C, 1 atm, pH 2 /P alkane = 9. The balance between the two functions was characterized by nPt/nA (nPt: number of accessible platinum atoms, nA: number of strong acid sites). Qualitatively the change as a function of nPt/nA of the catalytic characteristics is the same with n-hexane and with n-heptane: for low values of nPt/nA the activity per acid site is low, the stability is weak and monobranched isomers M, dibranched isomers B and cracking products C are formed directly from the reactant while for very high values the activity is optimal, the stability perfect and M, B, C are formed through a step-by-step process (“ideal” hydroisomerization catalyst). Quantitatively big differences exist. In particular in order to be “ideal”, a PtHY catalyst must have a much lower nPt/nA ratio for n-hexane isomerization. Moreover the reactivity of n-hexane is much lower than that of n-heptane. These results are interpreted in the light of the types of carbenium ions and of their rearrangement and scissions implied in n-hexane and n-heptane transformations.

Acetone Transformation over PtCu/H[Al]ZSM5 Catalysts. Effect of Copper Content

A series of bifunctional bimetallic PtCu/H[Al]ZSM5-type catalysts was prepared and acetone transformation was carried out over them at 160 °C, 1 atm, acetone/hydrogen molar ratio=3, and WHSV (weight of reactant injected per weight of catalyst per hour) ranging between 9.4 and 38 h−1. According to the results, as copper atomic fraction (X(Cu)) increases in the bimetallic catalyst, propane formation rate decreases and methyl isobutyl ketone (MIBK) formation rate increases until X(Cu) equals 0.40. These results suggest that active metallic centers for the olefin double bond hydrogenation in the α-β unsaturated ketone, which leads to the MIBK formation, do not appear to be those hydrogenating the acetone carbonyl double bond to form propane. When X(Cu)>0.40, MIBK formation rate considerably decreases, meaning that practically all platinum metallic centers have been passivated for those copper atomic fractions over the PtCu/H[Al]ZSM5-type catalyst. Therefore, catalysts begin to act only by means of the acid function, and as a result a logical increase in the mesityl oxide (MO) concentration occurs, a substance that is formed by means of an acid catalysis.

Hydroisomerization and Hydrocracking of Alkanes. 6. Influence of the Pore Structure on the Selectivity of Pt-Zeolite

Bifunctional metal-acid zeolites are used in numerous industrial processes in petroleum refining and in petrochemical industries1–3: hydrocracking, hydroisomerization of light alkanes or of C8 aromatics… On these catalysts alkane hydrocracking requires i) chemical steps on the metallic sites (dehydrogenation of alkanes and hydrogenation of olefinic intermediates) and on the acid sites (isomerization and cracking of olefins) and ii) diffusion steps of the intermediates from the metallic sites to the acid sites and vice versa 3. Therefore the activity, the stability and the selectivity of these bifungtjonal catalysts depend first on the acid and on the metallic functions3,4. We have recently shown in n-heptane transformation on PtHY catalysts that a definite correlation exists between the catalytic properties and the balance between the metallic and the acid functions characterized by nPt/nA, the ratio of the number of accessible platinum atoms to the number of strong acid sites (sites on which the heat of ammonia adsorption is greater than 100 kJ per mol). Thus for low values of this ratio the activity is low, the deactivation rapid and apparently n-heptane leads directly to all the isomerization and cracking products. For high values the activity is great, the deactivation very slow and n-heptane transforms successively into monobranched isomers, dibranched isomers and cracking products.