The story...

A molecule found in analysis of the WTC dust strongly suggests “that high-tech sol-gel thermite arson used on the WTC buildings”, suggests Professor Steven Jones:

... sol-gels to hold the thermite might leave telltale residue, 1,3-diphenlypropane (1,3-DPP)…
http://www.rsc.org/publishing/journals/CC/article.asp?doi=b310405b :
 “Pore size effects in the pyrolysis of 1,3-diphenylpropane confined in mesoporous silicas”
(Research by chemist Kevin Ryan)

• Analysis of the WTC dust showed:

“One molecule, described by the EPA's Erik Swartz, was present at levels "that dwarfed all others":

• “1,3-diphenylpropane. "We've never observed it in any sampling we've ever done," Swartz said.”
• http://www.newsday.com/news/health/nyhsair0911,0,471193.story?coll=ny-homepage-right-area
• http://oaspub.epa.gov/eims/xmlreport.display?deid=62021&z_chk=65088
• Large amounts of 1,3 diphenylpropane strongly suggest that high-tech sol-gel thermite arson used on the WTC buldings.
• We are further researching these matters…

FAQ: Questions and Answers
http://worldtradecentertruth.com/JonesAnswersQuestionsWorldTradeCenter.pdf

Our take...

Jones first tells us that this molecule was found in “analysis of the WTC dust”. That might lead you to believe that samples were taken from the ground, but this isn’t what happened, as the EPA link quoted by Jones reveals (our emphasis):

The September 11, 2001 attack on the World Trade Center (WTC) resulted in an intense fire and the subsequent, complete collapse of the two main structures and adjacent buildings, as well as significant damage to many surrounding buildings within and around the WTC complex. This 16-acre area has become known as Ground Zero. In response to this disaster, the US EPA quickly positioned air monitors and samplers around Ground Zero to characterize the resulting air plumes. One such instrument was a High Capacity Integrated Organic Gas and Particle (HiC IOGAP) sampler with a 2.5 mm cyclone for particle discrimination to collect semi-volatile gases and particles for speciation of organic compounds.
http://oaspub.epa.gov/eims/xmlreport.display?deid=62021&z_chk=65088

Samples were taken from the air, then. And according to the document “these emissions lasted for over three weeks (9/26/01-10/20/01) after the initial destruction of the WTC”. 

So thermite-related traces were found in the air more than three weeks later? That seems surprising, especially as earlier Jones takes time to reassure us that the amount of explosives required to bring down each tower wouldn’t be too significant:

...for a WTC tower, about 1000 pounds of explosives would be sufficient. This would only require an estimated 10 men (foreigners?) to install the cutter charges, mostly in the central core/ elevator shaft areas.

FAQ: Questions and Answers
http://worldtradecentertruth.com/JonesAnswersQuestionsWorldTradeCenter.pdf

Could there be an alternative explanation? Yes, as it happens, and it’s provided in both of the articles referenced by Professor Jones. Here’s the first quote, for instance, with the text that Jones left out in bold:

One molecule, described by the EPA's Erik Swartz, was present at levels "that dwarfed all others": 1,3-diphenylpropane. "We've never observed it in any sampling we've ever done," Swartz said. He said it was most likely produced by the plastic of tens of thousands of burning computers.
http://www.newsday.com/news/health/ny-hsair0911,0,471193.story?coll=ny-homepage-right-area

And here’s what the EPA article has to say:

In addition, the compound 1,3-diphenyl propane [1',1'-(1,3-propanediyl)bis-benzene] is found in significant concentrations. This species has not previously been reported from ambient sampling. It has been associated with polyvinyl chloride materials, which are believed to be in abundance at the WTC site.
http://oaspub.epa.gov/eims/xmlreport.display?deid=62021&z_chk=65088#top

Polyvinyl Chloride is PVC, a common form of plastic. A later paper on the same data mentions “other plastics”, and specified polystyrene as a possible source, too:

In addition, the compound 1,3-diphenylpropane- [ 1',1'-(1,3-propanediyl)bis-benzene] was observed, and to our knowledge, this species has not previously been reported from ambient sampling. It has been associated with polystyrene and other plastics, which are in abundance at the WTC site.
http://pubs.acs.org/cgi-bin/abstract.cgi/esthag/2003/37/i16/abs/es030356l.html

Polystyrene appears in more forms than you might think, although whether all of these produce 1,3-diphenylpropane is another matter:

The burning of polystyrene polymers - such as foam cups, meat trays, egg containers, yogurt and deli containers - releases styrene.
http://www.911ea.org/April12,2004.Testimony.Clarke.pdf

We know there were fires burning at ground zero for months after the attacks. It seems reasonable to assume that plastics and polystyrene were burning. The total volume of plastics capable of producing 1,3-diphenylpropane must surely have been considerably greater than the “sol-gels [required] to hold the [2,000 pounds of] thermite”, raising questions about Jones’ interpretation of this data,

Having the molecule produced by an ongoing reaction, burning plastics, would also explain why it was still present in the air “more than three weeks later”.

This, of course, is just speculation. What’s needed for a resolution is to deal with these alternative explanations, not by editing them out of existence, but instead exploring the questions raised. How much sol-gel would be required for Jones’ “1,000 pounds” of explosives per tower? How much 1,3-diphenylpropane might that produce, and is it plausible that would be detectable in air sampling some weeks later? Which plastics and other substances might also release 1,3-diphenylpropane as they burned, and in what quantities? And can any airbound “tell-tale residue” from sol-gel really be picked out from everything else that was going on in the debris pile? Or is the “burning plastics” explanation the most plausible answer here?