Pt-Quartz Catalysts

Experiments with these catalysts, from an old study by Schwab and Rudolph,3 showed that enantioselectivity occurs at the metal-quartz interface during enantioselective dehydrations of racemic 2-butanol. Klabunovskii and colleagues expanded on this idea and prepared a chiral Ni-quartz catalyst that 

One enantiomer in higher yield than other depending on quartz support 

Likely, this idea will continue to be explored because new chiral matrices are created frequently. Examples of published possible stable chiral matrices are chiral microporous cross-linked polymers5 and nanoscale chiral helical columns created from various inorganic materials.6 However, the interface problem will still exist. 

A 1996 work deposited four different catalytic metals on a p-cyclodextrin— epichlorohydrin copolymer to prepare Pd(Pt, Rh, Ru)-P-cyclodextrin copolymer catalysts.8 These were used to catalyze the asymmetric hydrogenations of the C=C bonds of trans-2-methyl-2-pentenoic acid, and dimethyl itaconate. 

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Increasing the amount of Ni on quartz somewhat diminished the rotation from -0.15 to -0.13°. On Pt-quartz catalysts 2-butanol produced a product with -0.14° at 353°C. Increasing the reaction temperature resulted in larger amounts of butenes and lower rotations. 

In the dehydration of 3-methylheptan-3-ol at 170°C Pt-/-quartz proved to be the most effective catalyst with an observed rotation reaching -0.26°. Thus it was shown that Pt-quartz catalysts give particularly large effects, presumably owing to uniform deposits of Pt-particles on the surfaces of the quartz. Table 2.5. below summarizes the main results received by Stankiewicz... 

Cu-, Ag-, and Pt-quartz catalysts were used for the asymmetric isomerization of the racemic mixture of methyloxirane (Scheme 2.3.). 

Hydrogenations of butan-2-one were carried out at 140-150°C. The optical rotation of the product was observed to be 0.10°, which fell over time to 0.02°. Other generally disappointing irregularities also occurred. Independent of the sign of the rotation of quartz, all products were levorotatory, and the optical activity disappeared in several hours. Quartz crystals without metal layers also catalyzed the dehydration of butan-2-ol at 350° and gave rotations of -0.075° and -0.085°, which after filtration of the products fell to -0.005°. Thus the products obtained with the Pt-/-quartz catalyst at 307°C had an initial rotation of -0.045° that fell to zero in 3 hrs. 

The apparatus used in the experiments is shown in figure 1 It is a slight modification of an apparatus designed for transient kinetic experiments [6]. The reactor was made by an U-shaped quartz tube (inner diameter 1,9 mm), In the H2-D2 experiments the oven was removed and the reactor was placed in a thermos with frozen C02 mixed with acetone to obtain -78 C An additional reactor, the equilibrium reactor", filled with 5.6 g commercial Pt/Al203 catalyst (EUROPT-3, CK 303), was used to achieve the equilibrium concentrations of H2, D2 and HD,... 

Figure 5. Hydrodesulfurization of thiophene over Pt/quartz(A), HZSM-5(B) and mechanically mixed (R/quartz(A) + HZSM-5(B)) catalysts at 40(TC.

The dispersion of Pt was determined from the amount of chemisorbed CO using pulse method. The experiments were carried out using a (Micromeritics 2910 AutoChem) instrument. The Pt modified catalysts were reduced with hydrogen in a U- shaped quartz tube and then cooled to 313 K under flow of He and CO pulse chemisorption was performed. More detail description of the R dispersion can be found in reference [11], The dispersion of R measured by CO chemisorption was the highest for R-MCM-41 catalyst prepared by ion-exchange method Table 1. 

Figure 24. dependence ofPi thickness of Pt/quartz plate catalyst on the NO rcduciion.(a)timc profile of Nj, (b)inlcgralcd amount of HjO and N2... 

The quartz made fluidized-bed reactor used for catalyst preparation was electrically heated a cyclone was incorporated into the freeboard of the reactor to prevent elutriation of fines. The support, which was fluidized by N2 was first thermally pretreated to remove physisorbed water. Then the vaporized metal acetyl acetonate was adsorbed on the support at 400 K at constant partial pressure in a flow of N2 for a ven period of time. The subsequent decomposition of the adsorbate was carried out by increasing the fluidized-bed temperature with a rate of 4 K/min. Pt was deposited in either N2 or air at 573 K Cr203 and V2O5 were deposited in air at 673 K. Pt catalysts which were decomposed in N2 were additionally treated in air at 573 K (Pt/Si02 catalysts) respectively at 773 K (Pt/Al203 catalysts) [4, 5]. 

Fig. 15-2. TOC 5000 instrument blank with (a) Pt-quartz wool catalyst, (b) with a Pt-silica beads catalyst.

That the heterogeneous and homogeneous processes in the oxidation of methane are closely related was shown in [128—130]. hr experiments with a sponge Ni catalyst and a Pt/Zr02 catalyst at a low contact time (<0.01 s) and temperatures above 600 °C, a hot spot arose and moved quickly towards the gas into the frontal layer and even beyond it. When a quartz wool wad was placed in front of the catalyst layer, its temperature ( 900 °C) exceeded that of the catalyst. Clearly, the catalyst generates reactive species (most likely free radicals) that diffuse upstream and initiate reactions in the gas phase in front of the... 

We have found new CO-tolerant catalysts by alloying Pt with a second, nonprecious, metal (Pt-Fe, Pt-Co, Pt-Ni, etc.) [Fujino, 1996 Watanabe et al., 1999 Igarashi et al., 2001]. In this section, we demonstrate the properties of these new alloy catalysts together with Pt-Ru alloy, based on voltammetric measurements, electrochemical quartz crystal microbalance (EQCM), electrochemical scanning tunneling microscopy (EC-STM), in situ Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). 

Catalyst. (0.3 wt% Pt/Ti02), 0.3 g yf catalyst suspended in 350 ml of water placed in an inner irradiator quartz cell Irradiated with high-pressure mercnry lamp (400 W)... 

Fig 7.8 Photocatalytic H2 evolution from an aqueous K2SO3 (0.25 mol/L)-Na2S (0.35 ml/L) solution (150 mL) under a simulated sun light irradiation over Pt (3.0 wt%)-loaded (AgIn)o,22Zni,56S2 solid solution heat-treated in a quartz ampule tube at 1123 K. Catalyst 0.3 g. solar simulator 300 W Xe short arc lamp with AM 1.5 filter. Reaction cell top window Pyrex cell. Irradiated area 33 cm Reprinted with permission from Ref. [165]. 

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... 

Fig. 14.2 Time course of H2 and 02 evolution from the Pt(0.3wt%)-TiO2 catalyst suspended in Na2C03 solution. Catalyst 0.3 g, Na2C03 80 g, water 350 ml, an inner-irradiation quartz cell, high-pressure Hg lamp (400 W). The evolved gas in the gas phase was pumped away at the moment indicated by the arrow(4-).

CO oxidation on a quartz wafer has been examined by the group of Weinberg [54]. They prepared catalyst samples by co-sputtering of the individual targets and by sol-gel techniques on a quartz wafer. The time for the preparation of a sputtered library is said to be only 1 h. A ternary library consisting of Pd, Pt and Rh exhibited sensitivity to C02 production, with the highest product yield in Pd-rich catalysts. 

Later, Veser [59] performed the reaction in a quartz-glass micro reactor with a diameter of 600 pm and 20 mm length (Figure 2.29). The ceramic housing of the reactor and the reactor itself were stable for temperatures exceeding 1100 °C. Again, a Pt wire of 150 pm diameter was used as a catalyst and electrically heated for startup. Residence times down to 50 ps could be achieved. 

A ternary library of noble metals is formed by deposition of solutions containing Pt, Pd and Rh by sol-gel techniques on a 3 in quartz wafer to form catalyst spots. 

Matsushita et al. (2007) subsequently demonstrated the ability to N-alkylate amines (Scheme 57) under continuous flow, again employing a quartz microreaction channel coated with a Ti02 or Pt-loaded Ti02 layer. As Table 28 illustrates, the illuminated specific surface area per unit of liquid attained within a microchannel is large even without taking into account the surface roughness of the catalyst however, it can be seen that a shallow reaction channel provides optimal photon efficiency. 

Participation of spillover hydrogen in the hydrodesulfurization of thiophene over Pt/HZSM-5 catalyst was assumed and examined. It was found that catalytic activity of PWSi02(quartz) mixed mechanically with HZSM-5 was higher than that obtained by simple addition of the data for the pure components(Figure 5). This implies that spillover hydrogen on Pt/HZSM-5 catalyst participates in the hydrodesulfurization of thiophene. The mechanism proposed is shown in Scheme 1. 

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